Acetylene Blowpipes—The design of a satisfactory blowpipe for use with acetylene had at first proved a matter of some difficulty, since the jet, like that of an ordinary self-luminous burner, usually exhibited a tendency to become choked with carbonaceous growths. But when acetylene had become available for various purposes at considerable pressure, after compression into porous matter as described in Chapter XI, the troubles were soon overcome; and a new form of blowpipe was constructed in which acetylene was consumed under pressure in conjunction with oxygen. The temperature given by this apparatus exceeds that of the familiar oxy- hydrogen blowpipe, because the actual combustible material is carbon instead of hydrogen. When 2 atoms of hydrogen unite with 1 of oxygen to form 1 molecule of gaseous water, about 59 large calories are evolved, and when 1 atom of solid amorphous carbon unites with 2 atoms of oxygen to form 1 molecule of carbon dioxide, 97.3 calories are evolved. In both cases, however, the heat attainable is limited by the fact that at certain temperatures hydrogen and oxygen refuse to combine to form water, and carbon and oxygen refuse to form carbon dioxide—in other words, water vapour and carbon dioxide dissociate and absorb heat in the process at certain moderately elevated temperatures. But when 1 atom of solid amorphous carbon unites with 1 atom of oxygen to form carbon monoxide, 29.1 [Footnote: Cf. Chapter VI., page 185.] large calories are produced, and carbon monoxide is capable of existence at much higher temperatures than either carbon dioxide or water vapour. In any gaseous hydrocarbon, again, the carbon exists in the gaseous state, and when 1 atom of the hypothetical gaseous carbon combines with 1 atom of oxygen to produce 1 molecule of carbon monoxide, 68.2 large calories are evolved. Thus while solid amorphous carbon emits more heat than a chemically equivalent quantity of hydrogen provided it is enabled to combine with its higher proportion of oxygen, it emits less if only carbon monoxide is formed; but a higher temperature can be attained in the latter case, because the carbon monoxide is more permanent or stable. Gaseous carbon, on the other hand, emits more heat than an equivalent quantity of hydrogen, [Footnote: In a blowpipe flame hydrogen can only burn to gaseous, not liquid, water.] even when it is only converted into the monoxide. In other words, a gaseous fuel which consists of hydrogen alone can only yield that temperature as a maximum at which the speed of the dissociation of the water vapour reaches that of the oxidation of the hydrogen; and were carbon dioxide the only oxide of carbon, a similar state of affairs would be ultimately reached in the flame of a carbonaceous gas. But since in the latter case the carbon dioxide does not tend to dissociate completely, but only to lose one atom of oxygen, above the limiting temperature for the formation of carbon dioxide, carbon monoxide is still produced, because there is less dissociating force opposed to its formation. Thus at ordinary temperatures the heat of combustion of acetylene is 315.7 calories; but at temperatures where water vapour and carbon dioxide no longer exist, there is lost to that quantity of 315.7 calories the heat of combustion of hydrogen (69.0) and twice that of carbon monoxide (68.2 x 2 = 136.4); so that above those critical temperatures, the heat of combustion of acetylene is only 315.7 - (69.0 + 136.4) = 110.3. [Footnote: When the heat of combustion of acetylene is quoted as 315.7 calories, it is understood that the water formed is condensed into the liquid state. If the water remains gaseous, as it must do in a flame, the heat of formation is reduced by about 10 calories. This does not affect the above calculation, because the heat of combustion of hydrogen when the water remains gaseous is similarly 10 calories less than 69, i.e., 59, as mentioned above in the text. Deleting the heat of liquefaction of water, the calculation referred to becomes 305.7 - (59.0 + l36.4) = 110.3 as before.] This value of 110.3 calories is clearly made up of the heat of formation of acetylene itself, and twice the heat of conversion of carbon into carbon monoxide, i.e., for diamond carbon, 58.1 + 26.1 x 2 = 110.3; or for amorphous carbon, 52.1 + 29.1 x 2 = 110.3. From the foregoing considerations, it may be inferred that the acetylene-oxygen blowpipe can be regarded as a device for burning gaseous carbon in oxygen; but were it possible to obtain carbon in the state of gas and so to lead it into a blowpipe, the acetylene apparatus should still be more powerful, because in it the temperature would be raised, not only by the heat of formation of carbon monoxide, but also by the heat attendant upon the dissociation of the acetylene which yields the carbon.
Acetylene requires 2.5 volumes of oxygen to burn it completely; but in the construction of an acetylene-oxygen blowpipe the proportion of oxygen is kept below this figure, viz., at 1.1 to 1.8 volumes, so that the deficiency is left to be made up from the surrounding air. Thus at the jet of the blowpipe the acetylene dissociates and its carbon is oxidised, at first no doubt to carbon monoxide only, but afterwards to carbon dioxide; and round the flame of the gaseous carbon is a comparatively cool, though absolutely very hot jacket of hydrogen burning to water vapour in a mixture of oxygen and air, which protects the inner zone from loss of heat. As just explained, theoretical grounds support the conclusions at which Fouche has arrived, viz., that the temperature of the acetylene-oxygen blowpipe flame is above that at which hydrogen will combine with oxygen to form water, and that it can only be exceeded by those found in a powerful electric furnace. As the hydrogen dissociated from the acetylene remains temporarily in the free state, the flame of the acetylene blowpipe, possesses strong reducing powers; and this, coupled probably with an intensity of heat which is practically otherwise unattainable, except by the aid of a high-tension electric current, should make the acetylene-oxygen blowpipe a most useful piece of apparatus for a large variety of metallurgical, chemical, and physical operations. In Fouche's earliest attempts to design an acetylene blowpipe, the gas was first saturated with a combustible vapour, such as that of petroleum spirit or ether, and the mixture was consumed with a blast of oxygen in an ordinary coal-gas blow-pipe. The apparatus worked fairly well, but gave a flame of varying character; it was capable of fusing iron, raised a pencil of lime to a more brilliant degree of incandescence than the eth-oxygen burner, and did not deposit carbon at the jet. The matter, however, was not pursued, as the blowpipe fed with undiluted acetylene took its place. The second apparatus constructed by Fouche was the high-pressure blowpipe, the theoretical aspect of which has already been studied. In this, acetylene passing through a water-seal from a cylinder where it is stored as a solution in acetone (cf. Chapter XI.), and oxygen coming from another cylinder, are each allowed to enter the blowpipe at a pressure of 118 to 157 inches of water column (i.e., 8.7 to 11.6 inches of mercury; 4.2 to 5.7 lb. per square inch, or 0.3 to 0.4 atmosphere). The gases mix in a chamber tightly packed with porous matter such as that which is employed in the original acetylene reservoir, and finally issue from a jet having a diameter of 1 millimetre at the necessary speed of 100 to 150 metres per second. Finding, however, that the need for having the acetylene under pressure somewhat limited the sphere of usefulness of his apparatus, Fouche finally designed a low-pressure blowpipe, in which only the oxygen requires to be in a state of compression, while the acetylene is drawn directly from any generator of the ordinary pattern that does not yield a gas contaminated with air. The oxygen passes through a reducing valve to lower the pressure under which it stands in the cylinder to that of 1 or 1.5 effective atmosphere, this amount being necessary to inject the acetylene and to give the previously mentioned speed of escape from the blowpipe orifice. The acetylene is led through a system of long narrow tubes to prevent it firing-back.
AUTOGENOUS SOLDERING AND WELDING.—The blowpipe is suitable for the welding and for the autogenous soldering or "burning" of wrought or cast iron, steel, or copper. An apparatus consuming from 600 to 1000 litres of acetylene per hour yields a flame whose inner zone is 10 to 15 millimetres long, and 3 to 4 millimetres in diameter; it is sufficiently powerful to burn iron sheets 8 to 9 millimetres thick. By increasing the supply of acetylene in proportion to that of the oxygen, the tip of the inner zone becomes strongly luminous, and the flame then tends to carburise iron; when the gases are so adjusted that this tip just disappears, the flame is at its best for heating iron and steel. The consumption of acetylene is about 75 litres per hour for each millimetre of thickness in the sheet treated, and the normal consumption of oxygen is 1.7 times as much; a joint 6 metres long can be burnt in 1 millimetre plate per hour, and one of 1.5 metres in 10 millimetre plate. In certain cases it is found economical to raise the metal to dull redness by other means, say with a portable forge of the usual description, or with a blowpipe consuming coal-gas and air. There are other forms of low- pressure blowpipe besides the Fouche, in some of which the oxygen also is supplied at low pressure. Apart from the use of cylinders of dissolved acetylene, which are extremely convenient and practically indispensable when the blowpipe has to be applied in confined spaces (as in repairing propeller shafts on ships in situ), acetylene generators are now made by several firms in a convenient transportable form for providing the gas for use in welding or autogenous soldering. It is generally supposed that the metal used as solder in soldering iron or steel by this method must be iron containing only a trifling proportion of carbon (such as Swedish iron), because the carbon of the acetylene carburises the metal, which is heated in the oxy-acetylene flame, and would thereby make ordinary steel too rich in carbon. But the extent to which the metal used is carburised in the flame depends, as has already been indicated, on the proper adjustment of the proportion of oxygen to acetylene. Oxy-acetylene autogenous soldering or welding is applicable to a great variety of work, among which may be mentioned repairs to shafts, locomotive frames, cylinders, and to joints in ships' frames, pipes, boilers, and rails. The use of the process is rapidly extending in engineering works generally. Generators for acetylene soldering or welding must be of ample size to meet the quickly fluctuating demands on them and must be provided with water-seals, and a washer or scrubber and filter capable of arresting all impurities held mechanically in the crude gas, and with a safety vent- pipe terminating in the open at a distance from the work in hand. The generator must be of a type which affords as little after-generation as possible, and should not need recharging while the blowpipe is in use. There should be a main tap on the pipe between the generator and the blowpipe. It does not appear conclusively established that the gas consumed should have been chemically purified, but a purifier of ample size and charged with efficient material is undoubtedly beneficial. The blowpipe must be designed so that it remains sufficiently cool to prevent polymerisation of the acetylene and deposition of the resultant particles of carbon or soot within it.
It is important to remember that if a diluent gas, such as nitrogen, is present, the superior calorific power of acetylene over nearly all gases should avail to keep the temperature of the flame more nearly up to the temperature at which hydrogen and oxygen cease to combine. Hence a blowpipe fed with air and acetylene would give a higher temperature than any ordinary (atmospheric) coal-gas blowpipe, just as, as has been explained in Chapter VI., an ordinary acetylene flame has a higher temperature than a coal-gas flame. It is likely that a blowpipe fed with "Linde-air" (oxygen diluted with less nitrogen than in the atmosphere) and acetylene would give as high a limelight effect as the oxy-hydrogen or oxy-coal-gas blowpipe.
Now that atmospheric or Bunsen burners for the consumption of acetylene for use in lighting by the incandescent system and in heating have been so much improved that they seem to be within measurable reach of a state of perfection, there appears to be but little use at the present time for a modified or diluted acetylene which formerly seemed likely to be valuable for heating and certain other purposes. Nevertheless, the facts relating to this so-called carburetted acetylene are in no way traversed by its failure to establish itself as an active competitor with simple acetylene for heating purposes, and since it is conceivable that the advantages which from the theoretical standpoint the carburetted gas undoubtedly possesses in certain directions may ultimately lead to its practical utilisation for special purposes, it has been deemed expedient to continue to give in this work an account of the principles underlying the production and application of carburetted acetylene.
It has already been explained that acetylene is comparatively a less efficient heating agent than it is an illuminating material, because, per unit of volume, its calorific power is not so much greater than that of coal-gas as is its illuminating capacity. It has also been shown that the high upper explosive limit of mixtures of acetylene and air—a limit so much higher than the corresponding figure with coal-gas and other gaseous fuels—renders its employment in atmospheric burners (either for lighting or for heating) somewhat troublesome, or dependent upon considerable skill in the design of the apparatus. If, therefore, either the upper explosive limit of acetylene could be reduced, or its calorific value increased (or both), by mixing with it some other gas or vapour which should not seriously affect its price and convenience as a self-luminous illuminant, acetylene would compare more favourably with coal-gas in its ready applicability to the most various purposes. Such a method has been suggested by Heil, and has been found successful on the Continent. It consists in adding to the acetylene a certain proportion of the vapour of a volatile hydrocarbon, so as to prepare what is called "carburetted acetylene." In all respects the method of making carburetted acetylene is identical with that of making "air-gas," which was outlined in Chapter I., viz., the acetylene coming from an ordinary generating plant is led over or through a mass of petroleum spirit, or other similar product, in a vessel which exposes the proper amount of superficial area to the passing gas. In all respects save one the character of the product is similar to that of air-gas, i.e., it is a mixture of a permanent gas with a vapour; the vapour may possibly condense in part within the mains if they are exposed to a falling temperature, and if the product is to be led any considerable distance, deposition of liquid may occur (conceivably followed by blockage of the mains) unless the proportion of vapour added to the gas is kept below a point governed by local climatic and similar conditions. But in one most important respect carburetted acetylene is totally different from air-gas: partial precipitation of spirit from air-gas removes more or less of the solitary useful constituent of the material, reducing its practical value, and causing the residue to approach or overpass its lower explosive limit (cf. Chapter I.); partial removal of spirit from carburetted acetylene only means a partial reconversion of the material into ordinary acetylene, increasing its natural illuminating power, lowering its calorific intensity somewhat, and causing the residue to have almost its primary high upper explosive limit, but essentially leaving its lower explosive limit unchanged. Thus while air-gas may conceivably become inefficient for every purpose if supplied from any distance in very cold weather, and may even pass into a dangerous explosive within the mains; carburetted acetylene can never become explosive, can only lose part of its special heating value, and will actually increase in illuminating power.
It is manifest that, like air-gas, carburetted acetylene is of somewhat indefinite composition, for the proportion of vapour, and the chemical nature of that vapour, may vary. 100 litres of acetylene will take up 40 grammes of petroleum spirit to yield 110 litres of carburetted acetylene evidently containing 9 per cent. of vapour, or 100 litres of acetylene may be made to absorb as much as 250 grammes of spirit yielding 200 litres of carburetted acetylene containing 50 per cent. of vapour; while the petroleum spirit may be replaced, if prices are suitable, by benzol or denatured alcohol.
The illuminating power of acetylene carburetted with petroleum spirit has been examined by Caro, whose average figures, worked out in British units, are:
ILLUMINATING POWER OF CARBURETTED ACETYLENE. HALF-FOOT BURNERS.
Self-luminous. Incandescent 1 litre = 1.00 candle. 1 litre = 3.04 candles. 1 cubic foot = 28.4 candles. 1 cubic foot = 86.2 candles. 1 candle = 1.00 litre. 1 candle = 0.33 litre. 1 candle = 0.035 cubic foot. 1 candle = 0.012 cubic foot.
Those results may be compared with those referring to air-gas, which emits in incandescent burners from 3.0 to 12.4 candles per cubic foot according to the amount of spirit added to the air and the temperature to which the gas is exposed.
The calorific values of carburetted acetylene (Caro), and those of other gaseous fuels are:
Large Calories per _ Cubic Foot. (Lewes) . 320 (Gand) . 403 Ordinary acetylene . . (Heil) . 365 _ _Mean . . 363
Maximum . 680 Carburetted acetylene . . Minimum . 467 (petroleum spirit) Mean . . 573
Carburetted acetylene (50 per cent. benzol by volume) 685 Carburetted acetylene (50 per cent. alcohol by volume) 364 Coal-gas (common, unenriched) . . . . . 150 Maximum . 178 Air-gas, self-luminous flame Minimum . 57 Mean . . . 114 Maximum . 26 Air-gas, non-luminous flame Minimum . 18 Mean . . . 22
Water-gas (Strache) from coke . . . . . 71 Mond gas (from bituminous coal) . . . . . 38 Semi-water-gas from coke or anthracite . . . 36 Generator (producer) gas . . . . . . 29
Besides its relatively low upper explosive limit, carburetted acetylene exhibits a higher temperature of ignition than ordinary acetylene, which makes it appreciably safer in presence of a naked light. It also possesses a somewhat lower flame temperature and a slower speed of propagation of the explosive wave when mixed with air. These data are:
Explosive Temperature. Limits. Degrees C. Explosive 19 mm. Tube. Explosive Wave. Metres per Of Igni- Second. Lower. Upper. tion. Of Flame. Acetylene (theoretical) - - - 1850-2420 - " (observed) 3.35 52.3 480 1630-2020 0.18-100 Carburetted from 2.5 10.2 582 1620 3.2 acetylene / . . to 5.4 30.0 720 1730 5.3 Carburetted acetylene 3.4 22.0 - 1820 1.3 (benzol) . . . / Carburetted acetylene 3.1 12.0 - 1610 1.1 (alcohol) . . . / Air-gas, self-luminous 15.0 50.0 - 1510-1520 - flame . . . . / Coal-gas . . . 7.9 19.1 600 - -
In making carburetted acetylene, the pressure given by the ordinary acetylene generator will be sufficient to drive the gas through the carburettor, and therefore there will be no expense involved beyond the cost of the spirit vaporised. Thus comparisons may fairly be made between ordinary and carburetted acetylene on the basis of material only, the expense of generating the original acetylene being also ignored. In Great Britain the prices of calcium carbide, petroleum spirit, and 90s benzol delivered in bulk in country places may be taken at 15L per ton, and 1s. per gallon respectively, petroleum spirit having a specific gravity of 0.700 and benzol of 0.88. On this basis, a unit volume (100 cubic metres) of plain acetylene costs 1135d., of "petrolised" acetylene containing 66 per cent. of acetylene costs 1277d., and of "benzolised" acetylene costs 1180d. In other words, 100 volumes of plain acetylene, 90 volumes of petrolised acetylene, and 96 volumes of benzolised acetylene are of equal pecuniary value. Employing the data given in previous tables, it appears that 38.5 candles can be won from plain acetylene in a self-luminous burner, and 103 candles therefrom in an incandescent burner at the same price as 25.5-29.1 and 78-87 candles can be obtained from carburetted acetylene; whence it follows that at English prices petrolised acetylene is more expensive as an illuminant in either system of combustion than the simple gas, while benzolised acetylene, burnt under the mantle only, is more nearly equal to the simple gas from a pecuniary aspect. But considering the calorific value, it appears that for a given sum of money only 363 calories can be obtained from plain acetylene, while petrolised acetylene yields 516, and benzolised acetylene 658; so that for all heating or cooking purposes (and also for driving small motors) carburetted acetylene exhibits a notable economy. Inasmuch as the partial saturation of acetylene with any combustible vapour is an operation of extreme simplicity, requiring no power or supervision beyond the occasional recharging of the carburettor, it is manifest that the original main coming from the generator supplying any large establishment where much warming, cooking (or motor driving) might conveniently be done with the gas could be divided within the plant-house, one branch supplying all, or nearly all, the lighting burners with plain acetylene, and the other branch communicating with a carburettor, so that all, or nearly all, the warming and cooking stoves (and the motor) should be supplied with the more economical carburetted acetylene. Since any water pump or similar apparatus would be in an outhouse or basement, and the most important heating stove (the cooker) be in the kitchen, such an arrangement would be neither complicated nor involve a costly duplication of pipes.
It follows from the fact that even a trifling proportion of vapour reduces the upper limit of explosibility of mixtures of acetylene with air, that the gas may be so lightly carburetted as not appreciably to suffer in illuminating power when consumed in self-luminous jets, and yet to burn satisfactorily in incandescent burners, even if it has been generated in an apparatus which introduces some air every time the operation of recharging is performed. To carry out this idea, Caro has suggested that 5 kilos. of petroleum spirit should be added to the generator water for every 50 cubic metres of gas evolved, i.e., 1 lb. per 160 cubic feet, or, say, 1 gallon per 1000 cubic feet, or per 200 lb. of carbide decomposed. Caro proposed this addition in the case of central installations supplying a district where the majority of the consumers burnt the gas in self-luminous jets, but where a few preferred the incandescent system; but it is clearly equally suitable for employment in all private plants of sufficient magnitude.
A lowering of the upper limit of explosibility is also produced by the presence of the acetone which remains in acetylene when obtained from a cylinder holding the compressed gas (cf. Chapter XI.). According to Wolff and Caro such gas usually carries with it from 30 to 60 grammes of acetone vapour per cubic metre, i.e., 1.27 grammes per cubic foot on an average; and this amount reduces the upper limit of explosibility by about 16 per cent., so that to this extent the gas behaves more smoothly in an incandescent burner of imperfect design.
Lepinay has described some experiments on the comparative technical value of ordinary acetylene, carburetted acetylene, denatured alcohol and petroleum spirit as fuels for small explosion engines. One particular motor of 3 (French) h.p. consumed 1150 grammes of petroleum spirit per hour at full load; but when it was supplied with carburetted acetylene its consumption fell to 150 litres of acetylene and 700 grammes of spirit (specific gravity 0.680). A 1-1/4 h.p. engine running light required 48 grammes of 90 per cent. alcohol per horse-power-hour and 66 litres of acetylene; at full load it took 220 grammes of alcohol and 110 litres of acetylene. A 6 h.p. engine at full load required 62 litres of acetylene carburetted with 197 grammes of petroleum spirit per horse-power-hour (uncorrected); while a similar motor fed with low-grade Taylor fuel-gas took 1260 litres per horse-power-hour, but on an average developed the same amount of power from 73 litres when 10 per cent. of acetylene was added to the gas. Lepinay found that with pure acetylene ignition of the charge was apt to be premature; and that while the consumption of carburetted acetylene in small motors still materially exceeded the theoretical, further economics could be attained, which, coupled with the smooth and regular running of an engine fed with the carburetted gas, made carburetted acetylene distinctly the better power-gas of the two.
COMPRESSED AND DISSOLVED ACETYLENE—MIXTURES WITH OTHER GASES
In all that was said in Chapters II., III., IV., and V. respecting the generation and employment of acetylene, it was assumed that the gas would be produced by the interaction of calcium carbide and water, either by the consumer himself, or in some central station delivering the acetylene throughout a neighbourhood in mains. But there are other methods of using the gas, which have now to be considered.
COMPRESSED ACETYLENE.—In the first place, like all other gases, acetylene is capable of compression, or even of conversion into the liquid state; for as a gas, the volume occupied by any given weight of it is not fixed, but varies inversely with the pressure under which it is stored. A steel cylinder, for instance, which is of such size as to hold a cubic foot of water, also holds a cubic foot of acetylene at atmospheric pressure, but holds 2 cubic feet if the gas is pumped into it to a pressure of 2 atmospheres, or 30 lb. per square inch; while by increasing the pressure to 21.53 atmospheres at 0 deg. C. (Ansdell, Willson and Suckert) the gas is liquefied, and the vessel may then contain 1 cubic foot of liquid acetylene, which is equal to some 400 cubic feet of gaseous acetylene at normal pressure. It is clear that for many purposes acetylene so compressed or liquefied would be convenient, for if the cylinders could be procured ready charged, all troubles incidental to generation would be avoided. The method, however, is not practically permissible; because, as pointed out in Chapters II. and VI., acetylene does not safely bear compression to a point exceeding 2 atmospheres; and the liability to spontaneous dissociation or explosion in presence of spark or severe blow, which is characteristic of compressed gaseous acetylene, is greatly enhanced if compression has been pushed to the point of liquefaction.
However, two methods of retaining the portability and convenience of compressed acetylene with complete safety have been discovered. In one, due to the researches of Claude and Hess, the gas is pumped under pressure into acetone, a combustible organic liquid of high solvent power, which boils at 56 deg. C. As the solvent capacity of most liquids for most gases rises with the pressure, a bottle partly filled with acetone may be charged with acetylene at considerable effective pressure until the vessel contains much more than its normal quantity of gas; and when the valve is opened the surplus escapes, ready for employment, leaving the acetone practically unaltered in composition or quantity, and fit to receive a fresh charge of gas. In comparison with liquefied acetylene, its solution in acetone under pressure is much safer; but since the acetone expands during absorption of gas, the bottle cannot be entirely filled with liquid, and therefore either at first, or during consumption (or both), above the level of the relatively safe solution, the cylinder contains a certain quantity of gaseous acetylene, which is compressed above its limit of safety. The other method consists in pumping acetylene under pressure into a cylinder apparently quite full of some highly porous solid matter, like charcoal, kieselguhr, unglazed brick, &c. This has the practical result that the gas is held under a high state of compression, or possibly as a liquid, in the minute crevices of the material, which are almost of insensible magnitude; or it may be regarded as stored in vessels whose diameter is less than that in which an explosive wave can be propagated (cf. Chapter VI.).
DISSOLVED ACETYLENE.—According to Fouche, the simple solution of acetylene in acetone has the same coefficient of expansion by heat as that of pure acetone, viz., 0.0015; the corresponding coefficient of liquefied acetylene is 0.007 (Fouche), or 0.00489 (Ansdell) i.e., three or five times as much. The specific gravity of liquid acetylene is 0.420 at 16.4 deg. C. (Ansdell), or 0.528 at 20.6 deg. C. (Willson and Suckert); while the density of acetylene dissolved in acetone is 0.71 at 15 deg. C. (Claude). The tension of liquefied acetylene is 21.53 atmospheres at 0 deg. C., and 39.76 atmospheres at 20.15 deg. C. (Ansdell); 21.53 at 0 deg. C., and 39.76 at 19.5 deg. C. (Willson and Suckert); or 26.5 at 0 deg. C., and 42.8 at 20.0 deg. C. (Villard). Averaging those results, it may be said that the tension rises from 23.2 atmospheres at 0 deg. C. to 40.77 at 20 deg. C., which is an increment of 1/26 or 0.88 atmosphere, per 1 deg. Centigrade; while, of course, liquefied acetylene cannot be kept at all at a temperature of 0 deg. unless the pressure is 21 atmospheres or upwards. The solution of acetylene in acetone can be stored at any pressure above or below that of the atmosphere, and the extent to which the pressure will rise as the temperature increases depends on the original pressure. Berthelot and Vieille have shown that when (a) 301 grammes of acetone are charged with 69 grammes of acetylene, a pressure of 6.74 atmospheres at 14.0 deg. C. rises to 10.55 atmospheres at 35.7 deg. C.; (b) 315 grammes of acetone are charged with 118 grammes of acetylene, a pressure of 12.25 atmospheres at 14.0 deg. C. rises to 19.46 at 36.0 deg. C.; (c) 315 grammes of acetone are charged with 203 grammes of acetylene, a pressure of 19.98 atmospheres at 13.0 deg. C. rises to 30.49 at 36.0 deg. C. Therefore in (a) the increase in pressure is 0.18 atmosphere, in (b) O.33 atmosphere, and in (c) 0.46 atmosphere per 1 deg. Centigrade within the temperature limits quoted. Taking case (b) as the normal, it follows that the increment in pressure per 1 deg. C. is 1/37 (usually quoted as 1/30); so that, measured as a proportion of the existing pressure, the pressure in a closed vessel containing a solution of acetylene in acetone increases nearly as much (though distinctly less) for a given rise in temperature as does the pressure in a similar vessel filled with liquefied acetylene, but the absolute increase is roughly only one-third with the solution as with the liquid, because the initial pressure under which the solution is stored is only one-half, or less, that at which the liquefied gas must exist.
Supposing, now, that acetylene contained in a closed vessel, either as compressed gas, as a solution in acetone, or as a liquid, were brought to explosion by spark or shock, the effects capable of production have to be considered. Berthelot and Vieille have shown that if gaseous acetylene is stored at a pressure of 11.23 kilogrammes per square centimetre, [Footnote: 1 kilo. per sq. cm. is almost identical with 1 atmosphere, or 15 lb. per sq. inch.] the pressure after explosion reaches 92.33 atmospheres on an average, which is an increase of 8.37 times the original figure; if the gas is stored at 21.13 atmospheres, the mean pressure after explosion is 213.15 atmospheres, or 10.13 times the original amount. If liquid acetylene is tested similarly, the original pressure, which must clearly be more than 21.53 atmospheres (Ansdell) at 0 deg. C., may rise to 5564 kilos, per square centimetre, as Berthelot and Vieille observed when a steel bomb having a capacity of 49 c.c. was charged with 18 grammes of liquefied acetylene. In the case of the solution in acetone, the magnitudes of the pressures set up are of two entirely different orders according as the original pressure 20 atmospheres or somewhat less; but apart from this, they vary considerably with the extent to which the vessel is filled with the liquid, and they also depend on whether the explosion is produced in the solution or in the gas space above. Taking the lower original pressure first, viz., 10 atmospheres, when a vessel was filled with solution to 33 per cent. of its capacity, the pressure after explosion reached about 95 atmospheres if the spark was applied to the gas space; but attained 117.4 atmospheres when the spark was applied to the acetone. When the vessel was filled 56 per cent. full, the pressures after explosion reached about 89, or 155 atmospheres, according as the gas or the liquid was treated with the spark. But when the original pressure was 20 atmospheres, and the vessel was filled to 35 per cent. of its actual capacity with solution, the final pressures ranged from 303 to 568 atmospheres when the gas was fired, and from 2000 to 5100 when the spark was applied to the acetone. Examining these figures carefully, it will be seen that the phenomena accompanying the explosion of a solution of acetylene in acetone resemble those of the explosion of compressed gaseous acetylene when the original pressure under which the solution is stored is about 10 atmospheres; but resemble those of the explosion of liquefied acetylene when the original pressure of the solution reaches 20 atmospheres, this being due to the fact that at an original pressure of 10 atmospheres the acetone itself does not explode, but, being exothermic, rather tends to decrease the severity of the explosion; whereas at an original pressure of 20 atmospheres the acetone does explode (or burn), and adds its heat of combustion to the heat evolved by the acetylene. Thus at 10 atmospheres the presence of the acetone is a source of safety; but at 20 atmospheres it becomes an extra danger.
Since sound steel cylinders may easily be constructed to boar a pressure of 250 atmospheres, but would be burst by a pressure considerably less than 5000 atmospheres, it appears that liquefied acetylene and its solution in acetone at a pressure of 20 atmospheres are quite unsafe; and it might also seem that both the solution at a pressure of 10 atmospheres and the simple gas compressed to the same limit should be safe. But there is an important difference here, in degree if not in kind, because, given a cylinder of known capacity containing (1) gaseous acetylene compressed to 10 atmospheres, or (2) containing the solution at the same pressure, if an explosion were to occur, in case (1) the whole contents would participate in the decomposition, whereas in case (2), as mentioned already, only the small quantity of gaseous acetylene above the solution would be dissociated.
It is manifest that of the three varieties of compressed acetylene now under consideration, the solution in acetone is the only one fit for general employment; but it exhibits the grave defects (a) that the pressure under which it is prepared must be so small that the pressure in the cylinders can never approach 20 atmospheres in the hottest weather or in the hottest situation to which they may be exposed, (b) that the gas does not escape smoothly enough to be convenient from large vessels unless those vessels are agitated, and (c) that the cylinders must always be used in a certain position with the valve at the top, lest part of the liquid should run out into the pipes. For these reasons the simple solution of acetylene in acetone has not become of industrial importance; but the processes of absorbing either the gas, or better still its solution in acetone, in porous matter have already achieved considerable success. Both methods have proved perfectly safe and trustworthy; but the combination of the acetone process with the porous matter makes the cylinders smaller per unit volume of acetylene they contain. Several varieties of solid matter appear to work satisfactorily, the only essential feature in their composition being that they shall possess a proper amount of porosity and be perfectly free from action upon the acetylene or the acetone (if present). Lime does attack acetone in time, and therefore it is not a suitable ingredient of the solid substance whenever acetylene is to be compressed in conjunction with the solvent; so that at present either a light brick earth which has a specific gravity of 0.5 is employed, or a mixture of charcoal with certain inorganic salts which has a density of 0.3, and can be introduced through a small aperture into the cylinder in a semi-fluid condition. Both materials possess a porosity of 80 per cent., that is to say, when a cylinder is apparently filled quite full, only 20 per cent, of the space is really occupied by the solid body, the remaining 80 per cent, being available for holding the liquid or the compressed gas. If all comparisons as to degree of explosibility and effects of explosion are omitted, an analogy may be drawn between liquefied acetylene or its compressed solution in acetone and nitroglycerin, while the gas or solution of the gas absorbed in porous matter resembles dynamite. Nitroglycerin is almost too treacherous a material to handle, but as an explosive (which in reason absorbed or dissolved acetylene is not) dynamite is safe, and even requires special arrangements to explode it.
In Paris, where the acetone process first found employment on a large scale, the company supplying portable cylinders to consumers uses large storage vessels filled, as above mentioned, apparently full of porous solid matter, and also charged to about 43 per cent, of their capacity with acetone, thus leaving about 37 per cent. of the apace for the expansion which occurs as the liquid takes up the gas. Acetylene is generated, purified, and thoroughly dried according to the usual methods; and it is then run through a double-action pump which compresses it first to a pressure of 3.5 kilos., next to a pressure of 3.5 x 3.5 = 12 kilos, per square centimetre, and finally drives it into the storage vessels. Compression is effected in two stages, because the process is accompanied by an evolution of much heat, which might cause the gas to explode during the operation; but since the pump is fitted with two cylinders, the acetylene can be cooled after the first compression. The storage vessels then contain 100 times their apparent volume of acetylene; for as the solubility of acetylene in acetone at ordinary temperature and pressure is about 25 volumes of gas in 1 of liquid, a vessel holding 100 volumes when empty takes up 25 x 43 = 1000 volumes of acetylene roughly at atmospheric pressure; which, as the pressure is approximately 10 atmospheres, becomes 1000 x 10 = 10,000 volumes per 100 normal capacity, or 100 times the capacity of the vessel in terms of water. From these large vessels, portable cylinders of various useful dimensions, similarly loaded with porous matter and acetone, are charged simply by placing them in mutual contact, thus allowing the pressure and the surplus gas to enter the small one; a process which has the advantage of renewing the small quantity of acetone vaporised from the consumers' cylinders as the acetylene is burnt (for acetone is somewhat volatile, cf. Chapter X.), so that only the storage vessels ever need to have fresh solvent introduced.
Where it is procurable, the use of acetylene compressed in this fashion is simplicity itself; for the cylinders have only to be connected with the house service-pipes through a reducing valve of ordinary construction, set to give the pressure which the burners require. When exhausted, the bottle is simply replaced by another. Manifestly, however, the cost of compression, the interest on the value of the cylinders, and the carriage, &c., make the compressed gas more expensive per unit of volume (or light) than acetylene locally generated from carbide and water; and indeed the value of the process does not lie so much in the direction of domestic illumination as in that of the lighting, and possibly driving, of vehicles and motor-cars—more especially in the illumination of such vehicles as travel constantly, or for business purposes, over rough road surfaces and perform mostly out-and-home journeys. Nevertheless, absorbed acetylene may claim close attention for one department of household illumination, viz., the portable table-lamp; for the base of such an apparatus might easily be constructed to imitate the acetone cylinder, and it could be charged by simple connexion with a larger one at intervals. In this way the size of the lamp for a given number of candle-hours would be reduced below that of any type of actual generator, and the troubles of after-generation, always more or less experienced in holderless generators, would be entirely done away with. Dissolved acetylene is also very useful for acetylene welding or autogenous soldering.
The advantages of compressed and absorbed acetylene depend on the small bulk and weight of the apparatus per unit of light, on the fact that no amount of agitation can affect the evolution of gas (as may happen with an ordinary acetylene generator), on the absence of any liquid which may freeze in winter, and on there being no need for skilled attention except when the cylinders are being changed. These vessels weigh between 2.5 and 3 kilos, per 1 litre capacity (normal) and since they are charged with 100 times their apparent volume of acetylene, they may be said to weigh 1 kilo, per 33 litres of available acetylene, or roughly 2 lb. per cubic foot, or, again, if half-foot burners are used, 2 lb. per 36 candle- hours. According to Fouche, if electricity obtained from lead accumulators is compared with acetylene on the basis of the weight of apparatus needed to evolve a certain quantify of light, 1 kilo, of acetylene cylinder is equal to 1.33 kilos, of lead accumulator with arc lamps, or to 4 kilos. of accumulator with glow lamps; and moreover the acetylene cylinder can be charged and discharged, broadly speaking, as quickly or as slowly as may be desired; while, it may be added, the same cylinder will serve one or more self-luminous jets, one or more incandescent burners, any number and variety of heating apparatus, simultaneously or consecutively, at any pressure which may be required. From the aspect of space occupied, dissolved acetylene is not so concentrated a source of artificial light as calcium carbide; for 1 volume of granulated carbide is capable of omitting as much light as 4 volumes of compressed gas; although, in practice, to the 1 volume of carbide must be added that of the apparatus in which it is decomposed.
LIQUEFIED ACETYLENE.—In most civilised countries the importation, manufacture, storage, and use of liquefied acetylene, or of the gas compressed to more than a fraction of one effective atmosphere, is quite properly prohibited by law. In Great Britain this has been done by an Order in Council dated November 26, 1897, which specifies 100 inches of water column as the maximum to which compression may be pushed. Power being retained, however, to exempt from the order any method of compressing acetylene that might be proved safe, the Home Secretary issued a subsequent Order on March 28, 1898, permitting oil-gas containing not more than 20 per cent, by volume of acetylene (see below) to be compressed to a degree not exceeding 150 lb. per square inch, i.e., to about 10 atmospheres, provided the gases are mixed together before compression; while a third Order, dated April 10, 1901, allows the compression of acetylene into cylinders filled as completely as possible with porous matter, with or without the presence of acetone, to a pressure not exceeding 150 lb. per square inch provided the cylinders themselves have been tested by hydraulic pressure for at least ten minutes to a pressure not less than double [Footnote: In France the cylinders are tested to six times and in Russia to five times their working pressure.] that which it is intended to use, provided the solid substance is similar in every respect to the samples deposited at the Home Office, provided its porosity does not exceed 80 per cent., provided air is excluded from every part of the apparatus before the gas is compressed, provided the quantity of acetone used (if used at all) is not sufficient to fill the porosity of the solid, provided the temperature is not permitted to rise during compression, and provided compression only takes place in premises approved by H.M.'s Inspectors of Explosives.
DILUTED ACETYLENE.—Acetylene is naturally capable of admixture or dilution with any other gas or vapour; and the operation may be regarded in either of two ways; (1) as a, means of improving the burning qualities of the acetylene itself, or (2) as a means of conferring upon some other gas increased luminosity. In the early days of the acetylene industry, generation was performed in so haphazard a fashion, purification so generally omitted, and the burners were so inefficient, that it was proposed to add to the gas a comparatively small proportion of some other gaseous fluid which should be capable of making it burn without deposition of carbon while not seriously impairing its latent illuminating power. One of the first diluents suggested was carbon dioxide (carbonic acid gas), because this gas is very easy and cheap to prepare; and because it was stated that acetylene would bear an addition of 5 or even 8 per cent, of carbon dioxide and yet develop its full degree of luminosity. This last assertion requires substantiation; for it is at least a grave theoretical error to add a non-inflammable gas to a combustible one, as is seen in the lower efficiency of all flames when burning in common air in comparison with that which they exhibit in oxygen; while from the practical aspect, so harmful is carbon dioxide in an illuminating gas, that coal-gas and carburetted water-gas are frequently most rigorously freed from it, because a certain gain in illuminating power may often thus be achieved more cheaply than by direct enrichment of the gas by addition of hydrocarbons. Being prepared from chalk and any cheap mineral acid, hydrochloric by preference, in the cold, carbon dioxide is so cheap that its price in comparison with that of acetylene is almost nil; and therefore, on the above assumption, 105 volumes of diluted acetylene might be made essentially for the same price as 100 volumes of neat acetylene, and according to supposition emit 5 per cent. more light per unit of volume.
It is reported that several railway trains in Austria are regularly lighted with acetylene containing 0.4 to 1.0 per cent. of carbon dioxide in order to prevent deposition of carbon at the burners. The gas is prepared according to a patent process which consists in adding a certain proportion of a "carbonate" to the generator water. In the United Kingdom, also, there are several installations supplying an acetylene diluted with carbon dioxide, the gas being produced by putting into that portion of a water-to-carbide generator which lies nearest to the water- supply some solid carbonate like chalk, and using a dilute acid to attack the material. Other inventors have proposed placing a solid acid, like oxalic, in the former part of a generator and decomposing it with a carbonate solution; or they have suggested putting into the generator a mixture of a solid acid and a solid soluble carbonate, and decomposing it with plain water.
Clearly, unless the apparatus in which such mixtures as these are intended to be prepared is designed with considerable care, the amount of carbon dioxide in the gas will be liable to vary, and may fall to zero. If any quantity of carbide present has been decomposed in the ordinary way, there will be free calcium hydroxide in the generator; and if the carbon dioxide comes into contact with this, it will be absorbed, unless sufficient acid is employed to convert the calcium carbonate (or hydroxide) into the corresponding normal salt of calcium. Similarly, during purification, a material containing any free lime would tend to remove the carbon dioxide, as would any substance which became alkaline by retaining the ammonia of the crude gas.
It cannot altogether be granted that the value of a process for diluting acetylene with carbon dioxide has been established, except in so far as the mere presence of the diluent may somewhat diminish the tendency of the acetylene to polymerise as it passes through a hot burner (cf. Chapter VIII.). Certainly as a fuel-gas the mixture would be less efficient, and the extra amount of carbon dioxide produced by each flame is not wholly to be ignored. Moreover, since properly generated and purified acetylene can be consumed in proper burners without trouble, all reason for introducing carbon dioxide has disappeared.
MIXTURES OF ACETYLENE AND AIR.—A further proposal for diluting acetylene was the addition to it of air. Apart from questions of explosibility, this method has the advantage over that of adding carbon dioxide that the air, though not inflammable, is, in virtue of its contained oxygen, a supporter of combustion, and is required in a flame; whereas carbon dioxide is not only not a supporter of combustion, but is actually a product thereof, and correspondingly more objectionable. According to some experiments carried out by Dufour, neat acetylene burnt under certain conditions evolved between 1.0 and 1.8 candle-power per litre- hour; a mixture of 1 volume of acetylene with 1 volume of air evolved 1.4 candle-power; a mixture of 1 volume of acetylene with 1.2 volumes of air, 2.25 candle-power; and a mixture of 1 volume of acetylene with 1.3 volumes of air, 2.70 candle-power per litre-hour of acetylene in the several mixtures. Averaging the figures, and calculating into terms of acetylene (only) burnt, Dufour found neat acetylene to develop 1.29 candle-power per litre-hour, and acetylene diluted with air to develop 1.51 candle-power. When, however, allowance is made for the cost and trouble of preparing such mixtures the advantage of the process disappears; and moreover it is accompanied by too grave risks, unless conducted on a largo scale and under most highly skilled supervision, to be fit for general employment.
Fouche, however, has since found the duty, per cubic foot of neat acetylene consumed in a twin injector burner at the most advantageous rate of 3.2 inches, to be as follows for mixtures with air in the proportions stated:
Percentage of air 0 17 27 33.5 Candles per cubic feet 38.4 36.0 32.8 26.0
At lower pressures, the duty of the acetylene when diluted appears to be relatively somewhat higher. Figures which have been published in regard to a mixture of 30 volumes of air and 70 volumes of acetylene obtained by a particular system of producing such a mixture, known as the "Molet- Boistelle," indicate that the admixture of air causes a slight increase in the illuminating duty obtained from the acetylene in burners of various sizes. The type of burner and the pressure employed in these experiments were not, however, stated. This system has been used at certain stations on the "Midi" railway in France. Nevertheless even where the admixture of air to acetylene is legally permissible, the risk of obtaining a really dangerous product and the nebulous character of the advantages attainable should preclude its adoption.
In Great Britain the manufacture, importation, storage, and use of acetylene mixed with air or oxygen, in all proportions and at all pressures, with or without the presence of other substances, is prohibited by an Order in Council dated July 1900; to which prohibition the mixture of acetylene and air that takes place in a burner or contrivance in which the mixture is intended to be burnt, and the admixture of air with acetylene that may unavoidably occur in the first use or recharging of an apparatus (usually a water-to-carbide generator), properly designed and constructed with a view to the production of pure acetylene, are the solitary exceptions.
MIXED CARBIDES.—In fact the only processes for diluting acetylene which possess real utility are that of adding vaporised petroleum spirit or benzene to the gas, as was described in Chapter X. under the name of carburetted acetylene, and one other possible method of obtaining a diluted acetylene directly from the gas-generator, to which a few words will now be devoted. [Footnote: Mixtures of acetylene with relatively large proportions of other illuminating gases, such as are referred to on subsequent pages, are also, from one aspect, forms of diluted acetylene.] Calcium carbide is only one particular specimen of a large number of similar metallic compounds, which can be prepared in the electric furnace, or otherwise. Some of those carbides yield acetylene when treated with water, some are not attacked, some give liquid products, and some yield methane, or mixtures of methane and hydrogen. Among the latter is manganese carbide. If, then, a mixture of manganese carbide and calcium carbide is put into an ordinary acetylene generator, the gas evolved will be a mixture of acetylene with methane and hydrogen in proportions depending upon the composition of the carbide mixture. It is clear that a suitable mixture of the carbides might be made by preparing them separately and bulking the whole in the desired proportions; while since manganese carbide can be won in the electric furnace, it might be feasible to charge into such a furnace a mixture of lime, coke, and manganese oxide calculated to yield a simple mixture of the carbides or a kind of double carbide. Following the lines which have been adopted in writing the present book, it is not proposed to discuss the possibility of making mixed carbides; but it may be said in brief that Brame and Lewes have carried out several experiments in this direction, using charges of lime and coke containing (a) up to 20 per cent. of manganese oxide, and (b) more than 60 per cent. of manganese oxide. In neither case did they succeed in obtaining a material which gave a mixture of acetylene and methane when treated with water; in case (a) they found the gas to be practically pure acetylene, so that the carbide must have been calcium carbide only; in case (b) the gas was mainly methane and hydrogen, so that the carbide must have been essentially that of manganese alone. Mixed charges containing between 20 and 60 per cent. of manganese oxide remain to be studied; but whether they would give mixed carbides or no, it would be perfectly simple to mix ready-made carbides of calcium and manganese together, if any demand for a diluted acetylene should arise on a sufficiently large scale. It is, however, somewhat difficult to appreciate the benefits to be obtained from forms of diluted acetylene other than those to which reference is made later in this chapter.
There is, nevertheless, one modification of calcium carbide which, in a small but important sphere, finds a useful _role_. It has been pointed out that a carbide containing much calcium phosphide is usually objectionable, because the gas evolved from it requires extra purification, and because there is the (somewhat unlikely) possibility that the acetylene obtained from such material before purification may be spontaneously inflammable. If, now, to the usual furnace charge of lime and coke a sufficient quantity of calcium phosphate is purposely added, it is possible to win a mixture of calcium phosphide and carbide, or, as Bradley, Read, and Jacobs call it, a "carbophosphide of calcium," having the formula Ca_5C_6P_2, which yields a spontaneously inflammable mixture of acetylene, gaseous phosphine, and liquid phosphine when treated with water, and which, therefore, automatically gives a flame when brought into contact with the liquid. The value of this material will be described in Chapter XIII.
GAS-ENRICHING.—Other methods of diluting acetylene consist in adding a comparatively small proportion of it to some other gas, and may be considered rather as processes for enriching that other gas with acetylene. Provided the second gas is well chosen, such mixtures exhibit properties which render them peculiarly valuable for special purposes. They have, usually, a far lower upper limit of explosibility than that of neat acetylene, and they admit of safe compression to an extent greatly exceeding that of acetylene itself, while they do not lose illuminating power on compression. The second characteristic is most important, and depends on the phenomena of "partial pressure," which have been referred to in Chapter VI. When a single gas is stored at atmospheric pressure, it is insensibly withstanding on all sides and in all directions a pressure of roughly 15 lb. per square inch, which is the weight of the atmosphere at sea-level; and when a mixture of two gases, X and Y, in equal volumes is similarly stored it, regarded as an entity, is also supporting a pressure of 15 lb. per square inch. But in every 1 volume of that mixture there is only half a volume of X and Y each; and, ignoring the presence of its partner, each half-volume is evenly distributed throughout a space of 1 volume. But since the volume of a gas stands in inverse ratio to the pressure under which it is stored, the half-volume of X in the 1 volume of X + Y apparently stands at a pressure of half an atmosphere, for it has expanded till it fills, from a chemical and physical aspect, the space of 1 volume: suitable tests proving that it exhibits the properties which a gas stored at a pressure of half an atmosphere should do. Therefore, in the mixture under consideration, X and Y are both said to be at a "partial pressure" of half an atmosphere, which is manifestly 7.5 lb. per square inch. Clearly, when a gas is an entity (either an element or one single chemical compound) partial and total pressure are identical. Now, it has been shown that acetylene ceases to be a safe gas to handle when it is stored at a pressure of 2 atmospheres; but the limit of safety really occurs when the gas is stored at a partial pressure of 2 atmospheres. Neat acetylene, accordingly, cannot be compressed above the mark 30 lb. shown on a pressure gauge; but diluted acetylene (if the diluent is suitable) may be compressed in safety till the partial pressure of the acetylene itself reaches 2 atmospheres. For instance, a mixture of equal volumes of X and Y (X being acetylene) contains X at a partial pressure of half the total pressure, and may therefore be compressed to (2 / 1/2 =) 4 atmospheres before X reaches the partial pressure of 2 atmospheres; and therewith the mixture is brought just to the limit of safety, any effect of Y one way or the other being neglected. Similarly, a mixture of 1 volume of acetylene with 4 volumes of Y may be safely compressed to a pressure of (2 / 1/5 =) 10 atmospheres, or, broadly, a mixture in which the percentage of acetylene is x may be safely compressed to a pressure not exceeding (2 / x/100) atmospheres. This fact permits acetylene after proper dilution to be compressed in the same fashion as is allowable in the case of the dissolved and absorbed gas described above.
If the latent illuminating power of acetylene is not to be wasted, the diluent must not be selected without thought. Acetylene burns with a very hot flame, the luminosity of which is seriously decreased if the temperature is lowered. As mentioned in Chapter VIII., this may be done by allowing too much air to enter the flame; but it may also be effected to a certain extent by mixing with the acetylene before combustion some combustible gas or vapour which burns at a lower temperature than acetylene itself. Manifestly, therefore, the ideal diluent for acetylene is a substance which possesses as high a flame temperature as acetylene and a certain degree of intrinsic illuminating power, while the lower the flame temperature of the diluent and the less its intrinsic illuminating power, the less efficiently will the acetylene act as an enriching material. According to Love, Hempel, Wedding, and others, if acetylene is mixed with coal-gas in amounts up to 8 per cent. or thereabouts, the illuminating power of the mixture increases about 1 candle for every 1 per cent. of acetylene present: a fact which is usually expressed by saying that with coal-gas the enrichment value of acetylene is 1 candle per 1 per cent. Above 8 per cent., the enrichment value of acetylene rises, Love having found an increase in illuminating power, for each 1 per cent. of acetylene in the mixture, of 1.42 candles with 11.28 per cent. of acetylene; and of 1.54 candles with 17.62 per cent. of acetylene. Theoretically, if the illuminating power of acetylene is taken at 240 candles, its enrichment value should be (240 / 100 =) 2.4 candles per 1 per cent.; and since, in the case of coal-gas, its actual enrichment value falls seriously below this figure, it is clear that coal-gas is not an economical diluent for it. Moreover, coal-gas can be enriched by other methods much more cheaply than with acetylene. Simple ("blue") water-gas, according to Love, requires more than 10 per cent. of acetylene to be added to it before a luminous flame is produced; while a mixture of 20.3 per cent. of acetylene and 79.7 per cent. of water-gas had an illuminating power of 15.47 candles. Every addition to the proportion of acetylene when it amounted to 20 per cent. and upwards of the mixture had a very appreciable effect on the illuminating power of the latter. Thus with 27.84 per cent. of acetylene, the illuminating power of the mixture was 40.87 candles; with 38.00 per cent. of acetylene it was 73.96 candles. Acetylene would not be an economical agent to employ in order to render water-gas an illuminating gas of about the quality of coal-gas, but the economy of enrichment of water-gas by acetylene increases rapidly with the degree of enrichment demanded of it. Carburetted water-gas which, after compression under 16 atmospheres pressure, had an illuminating power of about 17.5 candles, was enriched by additions of acetylene. 4.5 per cent. of acetylene in the mixture gave an illuminating power of 22.69 candles; 8.4 per cent., 29.54 candles; 11.21 per cent., 35.05 candles; 15.06 per cent., 42.19 candles; and 21.44 per cent., 52.61 candles. It is therefore evident that the effect of additions of acetylene on the illuminating power of carburetted water-gas is of the same order as its effect on coal-gas. The enrichment value of the acetylene increases with its proportion in the mixture; but only when the proportion becomes quite considerable, and, therefore, the gas of high illuminating power, does enrichment by acetylene become economical. Methane (marsh-gas), owing to its comparatively high flame temperature, and to the fact that it has an intrinsic, if small, illuminating power, is a better diluent of acetylene than carbon monoxide or hydrogen, in that it preserves to a greater extent the illuminative value of the acetylene.
Actually comparisons of the effect of additions of various proportions of a richly illuminating gas, such as acetylene, on the illuminative value of a gas which has little or no inherent illuminating power, are largely vitiated by the want of any systematic method for arriving at the representative illuminative value of any illuminating gas. A statement that the illuminating power of a gas is x candles is, strictly speaking, incomplete, unless it is supplemented by the information that the gas during testing was burnt (1) in a specified type of burner, and (2) either at a specified fixed rate of consumption or so as to afford a light of a certain specified intensity. There is no general agreement, even in respect of the statutory testing of the illuminating power of coal-gas supplies, as to the observance of uniform conditions of burning of the gas under test, and in regard to more highly illuminating gases there is even greater diversity of conditions. Hence figures such as those quoted above for the enrichment value of acetylene inevitably show a certain want of harmony which is in reality due to the imperfection or incompleteness of the modes of testing employed. Relatively to another, one gas appears advantageously merely in virtue of the conditions of assessing illuminating power having been more favourable to it. Therefore enrichment values, such as those given, must always be regarded as only approximately trustworthy in instituting comparisons between either different diluent gases or different enriching agents.
ACETYLENE MIXTURES FOR RAILWAY-CARRIAGE LIGHTING.—In modern practice, the gases which are most commonly employed for diluents of acetylene, under the conditions now being considered, are cannel-coal gas (in France) and oil-gas (elsewhere). Fowler has made a series of observations on the illuminating value of mixtures of oil-gas and acetylene. 13.41 per cent. of acetylene improved the illuminating power of oil-gas from 43 to 49 candles. Thirty-nine-candle-power oil-gas had its illuminating power raised to about 60 candles by an admixture of 20 per cent. of acetylene, to about 80 candles by 40 per cent. of acetylene, and to about 110 candles by 60 per cent. of acetylene. The difficulty of employing mixtures fairly rich in acetylene, or pure acetylene, for railway- carriage lighting, lies in the poor efficiency of the small burners which yield from such rich gas a light of 15 to 20 candle-power, such as is suitable for the purpose. For the lighting of railway carriages it is seldom deemed necessary to have a flame of more than 20 candle-power, and it is somewhat difficult to obtain such a flame from oil-gas mixtures rich in acetylene, unless the illuminative value of the gas is wasted to a considerable extent. According to Bunte, 15 volumes of coal-gas, 8 volumes of German oil-gas, and 1.5 volumes of acetylene all yield an equal amount of light; from which it follows that 1 volume of acetylene is equivalent to 5.3 volumes of German oil-gas.
A lengthy series of experiments upon the illuminating power of mixtures of oil-gas and acetylene in proportions ranging between 10 and 50 per cent. of the latter, consumed in different burners and at different pressures, has been carried out by Borck, of the German State Railway Department. The figures show that per unit of volume such mixtures may give anything up to 6.75 times the light evolved by pure oil-gas; but that the latent illuminating power of the acetylene is less advantageously developed if too much of it is employed. As 20 per cent. of acetylene is the highest proportion which may be legally added to oil- gas in this country, Borck's results for that mixture may be studied:
Propor- Consump- Consump- tionate Kind of No. of Pres- tion per Candle- tion per Illum- Burner. Burner sure. Hour. Power. Candle- inating mm. Litres. Hour. Power Litres. to Pure Oil-Gas. Bray 00 42 82 56.2 1.15 3.38 " 000 35 54 28.3 1.91 4.92 " 0000 35 43.3 16 2.71 4.90 Oil-gas burner 15 24 21 7.25 2.89 4.53 " " 30 15 22 10.5 2.09 3.57 " " 40 16 33.5 20.2 1.65 3.01 " " 60 33 73 45.2 1.62 3.37 The oil-gas from which this mixture was prepared showing: Bray 00 34 73.5 16.6 4.42 ... " 000 30 48 6.89 6.96 ... " 0000 28 39 3.26 11.6 ... Oil-gas burner 15 21 19 1.6 11.8 ... " " 30 14 21.5 2.94 7.31 ... " " 40 15 33 6.7 4.92 ... " " 60 25 60 13.4 4.40 ...
It will be seen that the original oil-gas, when compressed to 10 atmospheres, gave a light of 1 candle-hour for an average consumption of 7.66 litres in the Bray burners, and for a consumption of 7.11 litres in the ordinary German oil-gas jets; while the mixture containing 20 per cent. of acetylene evolved the same amount of light for a consumption of 2.02 litres in Bray burners, or of 2.06 litres in the oil-gas jets. Again, taking No. 40 as the most popular and useful size of burner, 1 volume of acetylene oil-gas may be said to be equal to 3 volumes of simple oil-gas, which is the value assigned to the mixture by the German Government officials, who, at the prices ruling there, hold the mixture to be twice as expensive as plain oil-gas per unit of volume, which means that for a given outlay 50 per cent. more light may be obtained from acetylene oil-gas than from oil-gas alone.
This comparison of cost is not applicable, as it stands, to compressed oil-gas, with and without enrichment by acetylene, in this country, owing to the oils from which oil-gas is made being much cheaper and of better quality here than in Germany, where a heavy duty is imposed on imported petroleum. Oil-gas as made from Scotch and other good quality gas-oil in this country, usually has, after compression, an illuminating duty of about 8 candles per cubic foot, which is about double that of the compressed German oil-gas as examined by Borck.
Hence the following table, containing a summary of results obtained by H. Fowler with compressed oil-gas, as used on English railways, must be accepted rather than the foregoing, in so far as conditions prevailing in this country are concerned. It likewise refers to a mixture of oil-gas and acetylene containing 20 per cent. of acetylene.
Ratio of Consumption Candles per Illuminating Burner. Pressure. per Hour. Candle Cubic Foot Power to that Inches. Cubic Feet. Power. per Hour. of Oil-gas  in the same Burner. Oil-gas . . 0.7 0.98 12.5 12.72 1.65 Bray 000 . 0.7 1.17 14.4 12.30 1.57 " 0000 . 0.7 0.97 10.4 10.74 1.41 " 00000 0.7 0.78 5.6 7.16 1.08 " 000000 0.7 0.55 1.9 3.52 1.14
[Footnote 1: Data relating to the relative pecuniary values of acetylene (carburetted or not), coal-gas, paraffin, and electricity as heating or illuminating agents, are frequently presented to British readers after simple recalculation into English equivalents of the figures which obtain in France and Germany. Such a method of procedure is utterly incorrect, as it ignores the higher prices of coal, coal-gas, and especially petroleum products on the Continent of Europe, which arise partly from geographical, but mainly from political causes.]
The mixture was tried also at higher pressures in the same burners, but with less favourable results in regard to the duty realised. The oil-gas was also tried at various pressures, and the most favourable result is taken for computing the ratio in the last column. It is evident from this table that 1 volume of this acetylene-oil-gas mixture is equal at the most to 1.65 volume of the simple oil-gas. Whether the mixture will prove cheaper under particular conditions must depend on the relative prices of gas-oil and calcium carbide at the works where the gas is made and compressed. At the prevailing prices in most parts of Britain, simple oil-gas is slightly cheaper, but an appreciable rise in the price of gas- oil would render the mixture with acetylene the cheaper illuminant. The fact remains, however, that per unit weight or volume of cylinder into which the gas is compressed, acetylene oil-gas evolves a higher candle- power, or the same candle-power for a longer period, than simple, unenriched British oil-gas. Latterly, however, the incandescent mantle has found application for railway-carriage lighting, and poorer compressed gases have thereby been rendered available. Thus coal-gas, to which a small proportion of acetylene has been added, may advantageously displace the richer oil-gas and acetylene mixtures.
Patents have been taken out by Schwander for the preparation of a mixture of acetylene, air, and vaporised petroleum spirit. A current of naturally damp, or artificially moistened, air is led over or through a mass of calcium carbide, whereby the moisture is replaced by an equivalent quantity of acetylene; and this mixture of acetylene and air is carburetted by passing it through a vessel of petroleum spirit in the manner adopted with air-gas. No details as to the composition, illuminating power, and calorific values of the gas so made have been published. It would clearly tend to be of highly indefinite constitution and might range between what would be virtually inferior carburetted acetylene, and a low-grade air-gas. It is also doubtful whether the combustion of such gas would not be accompanied by too grave risks to render the process useful.
There are sundry uses for acetylene, and to some extent for carbide, which are not included in what has been said in previous chapters of this book; and to them a few words may be devoted.
In orchards and market gardens enormous damage is frequently done to the crops by the ravages of caterpillars of numerous species. These caterpillars cannot be caught by hand, and hitherto it has proved exceedingly difficult to cope with them. However, when they have changed into the perfect state, the corresponding butterflies and moths, like most other winged insects, are strongly attracted by a bright light. As acetylene can easily be burnt in a portable apparatus, and as the burners can be supplied with gas at such comparatively high pressure that the flames are capable of withstanding sharp gusts of wind even when not protected by glass, the brilliant light given by acetylene forms an excellent method of destroying the insects before they have had time to lay their eggs. Two methods of using the light have been tried with astonishing success: in one a naked flame is supported within some receptacle, such as a barrel with one end knocked out, the interior of which is painted heavily with treacle; in the other the flame is supported over an open dish filled with some cheap heavy oil (or perhaps treacle would do equally well). In the first case the insects are attracted by the light and are caught by the adhesive surfaces; in the second they are attracted and singed, and then drowned in, or caught by, the liquid. Either a well-made, powerful, vehicular lamp with its bull's- eye (if any) removed could be used for this purpose, or a portable generator of any kind might be connected with the burner through a flexible tube. It is necessary that the lights should be lit just before dusk when the weather is fine and the nights dark, and for some twenty evenings in June or July, exactly at the period of the year when the perfect insects are coming into existence. In some of the vineyards of Beaujolais, in France, where great havoc has been wrought by the pyralid, a set of 10-candle-power lamps were put up during July 1901, at distances of 150 yards apart, using generators containing 6 oz. of carbide, and dishes filled with water and petroleum 18 or 20 inches in diameter. In eighteen nights, some twenty lamps being employed, the total catch of insects was 170,000, or an average of 3200 per lamp per night. At French prices, the cost is reported to have been 8 centimes per night, or 32 centimes per hectare (2.5 acres). In Germany, where school children are occasionally paid for destroying noxious moths, two acetylene lamps burning for twelve evenings succeeded in catching twice as many insects as the whole juvenile population of a village during August 1902. A similar process has been recommended for the destruction of the malarial mosquito, and should prove of great service to mankind in infected districts. The superiority of acetylene in respect of brilliancy and portability will at once suggest its employment as the illuminant in the "light" moth-traps which entomologists use for entrapping moths. In these traps, the insects, attracted by the light, flutter down panes of glass, so inclined that ultimate escape is improbable; while they are protected from injury through contact with the flame by moans of an intervening sheet of glass.
Methods of spraying with carbide dust have been found useful in treating mildew in vines; while a process of burying small quantities of carbide at the roots has proved highly efficacious in exterminating phylloxera in the French and Spanish vineyards. It was originally believed that the impurities of the slowly formed acetylene, the phosphine in particular, acted as toxic agents upon the phylloxera; and therefore carbide containing an extra amount of decomposable phosphides was specially manufactured for the vine-growers. But more recently it has been argued, with some show of reason, that the acetylene itself plays a part in the process, the effects produced being said to be too great to be ascribed wholly to the phosphine. It is well known that many hydrocarbon vapours, such as the vapour of benzene or of naphthalene, have a highly toxic action on low organisms, and the destructive effect of acetylene on phylloxera may be akin to this action.
As gaseous acetylene will bear a certain amount of pressure in safety—a pressure falling somewhat short of one effective atmosphere—and as pressure naturally rises in a generating apparatus where calcium carbide reacts with water, it becomes possible to use this pressure as a source of energy for several purposes. The pressure of the gas may, in fact, be employed either to force a stream of liquid through a pipe, or to propel certain mechanism. An apparatus has been constructed in France on the lines of some portable fire-extinguishing appliances in which the pressure set up by the evolution of acetylene in a closed space produces a spray of water charged with lime and gas under the pressure obtaining; the liquid being thrown over growing vines or other plants in order to destroy parasitic and other forms of life. The apparatus consists of a metal cylinder fitted with straps so that it can be carried by man or beast. At one end it has an attachment for a flexible pipe, at the other end a perforated basket for carbide introduced and withdrawn through a "man-hole" that can be tightly closed. The cylinder is filled with water to a point just below the bottom of the basket when the basket is uppermost; the carbide charge is then inserted, and the cover fastened down. As long as the cylinder is carried in the same position, no reaction between the carbide and the water occurs, and consequently no pressure arises; but on inverting the vessel, the carbide is wetted, and acetylene is liberated in the interior. On opening the cock on the outlet pipe, a stream of liquid issues and may be directed as required. By charging the cylinder in the first place with a solution of copper sulphate, the liquid ejected becomes a solution and suspension of copper and calcium salts and hydroxides, resembling "Bordeaux mixture," and may be employed as such. In addition, it is saturated with acetylene which adds to its value as a germicide.
The effective gas pressure set up in a closed generator has also been employed in Italy to drive a gas-turbine, and so to produce motion. The plant has been designed for use in lighthouses where acetylene is burnt, and where a revolving or flashing light is required. The gas outlet from a suitably arranged generator communicates with the inlet of a gas- turbine, and the outlet of the turbine is connected to a pipe leading to the acetylene burners. The motion of the turbine is employed to rotate screens, coloured glasses, or any desired optical arrangements round the flames; or, in other situations, periodically to open and close a cock on the gas-main leading to the burners. In the latter case, a pilot flame fed separately is always alight, and serves to ignite the gas issuing from the main burners when the cock is opened.
Another use for acetylene, which is only dependent upon a suitably lowered price for carbide to become of some importance, consists in the preparation of a black pigment to replace ordinary lampblack. One method for this purpose has been elaborated by Hubou. Acetylene is prepared from carbide smalls or good carbide, according to price, and the gas is pumped into small steel cylinders to a pressure of 2 atmospheres. An electric spark is then passed, and the gas, standing at its limit of safety, immediately dissociates, yielding a quantitative amount of hydrogen and free carbon. The hydrogen is drawn off, collected in holders, and used for any convenient purpose; the carbon is withdrawn from the vessel, and is ready for sale. At present the pigment is much too expensive, at least in British conditions, to be available in the manufacture of black paint; but its price would justify its employment in the preparation of the best grades of printers' ink. One of the authors has examined an average sample and has found it fully equal in every way to blacks, such as those termed "spirit blacks," which fetch a price considerably above their real value. It has a pure black cast of tint, is free from greasy matter, and can therefore easily be ground into water, or into linseed oil without interfering with the drying properties of the latter. Acetylene black has also been tried in calico printing, and has given far better results in tone and strength than other blacks per unit weight of pigment. It may be added that the actual yield of pigment from creosote oils, the commonest raw material for the preparation of lampblack ("vegetable black"), seldom exceeds 20 or 25 per cent., although the oil itself contains some 80 per cent, of carbon. The yield from acetylene is clearly about 90 per cent., or from calcium carbide nearly 37.5 per cent, of the original weight.
An objection urged against the Hubou process is that only small quantities of the gas can be treated with the spark at one time; if the cylinders are too large, it is stated, tarry by-products are formed. A second method of preparing lampblack (or graphite) from acetylene is that devised by Frank, and depends on utilising the reactions between carbon monoxide or dioxide and acetylene or calcium carbide, which have already been sketched in Chapter VI. When acetylene is employed, the yield is pure carbon, for the only by-product is water vapour; but if the carbide process is adopted, the carbon remains mixed with calcium oxide. Possibly such a material as Frank's carbide process would give, viz., 36 parts by weight of carbon mixed with 56 parts of quicklime or 60 parts of carbon mixed with 112 parts of quicklime, might answer the purpose of a pigment in some black paints where the amount of ash left on ignition is not subject to specification. Naturally, however, the lime might be washed away from the carbon by treatment with hydrochloric acid; but the cost of such a purifying operation would probably render the residual pigment too expensive to be of much service except (conceivably) in the manufacture of certain grades of printers' ink, for which purpose it might compete with the carbon obtainable by the Hubou process already referred to.
Acetylene tetrachloride, or tetrachlorethane, C_2H_2Cl_4, is now produced for sale as a solvent for chlorine, sulphur, phosphorus, and organic substances such as fats. It may be obtained by the direct combination of acetylene and chlorine as explained in Chapter VI., but the liability of the reaction to take place with explosive violence would preclude the direct application of it on a commercial scale. Processes free from such risk have now, however, been devised for the production of tetrachlorethane. One patented by the Salzbergwerk Neu-Stassfurt consists in passing acetylene into a mixture of finely divided iron and chloride of sulphur. The iron acts as a catalytic. The liquid is kept cool, and as soon as the acetylene passes through unabsorbed, its introduction is stopped and chlorine is passed in. Acetylene and chlorine are then passed in alternately until the liquid finally is saturated with acetylene. The tetrachlorethane, boiling at 147 deg. C., is then distilled off, and the residual sulphur is reconverted to the chloride for use again in the process. A similar process in which the chlorine is used in excess is applicable also to the production of hexachlorethane.
Dependent upon price, again, are several uses for calcium carbide as a metallurgical or reducing reagent; but as those are uses for carbide only as distinguished from acetylene, they do not fall within the purview of the present book.
When discussing, in Chapter III., methods for disposing of the lime sludge coming from an acetylene generator, it was stated that on occasion a use could be found for this material. If the carbide has been entirely decomposed in an apparatus free from overheating, the waste lime is recovered as a solid mass or as a cream of lime practically pure white in colour. Sometimes, however, as explained in Chapter II., the lime sludge is of a bluish grey tint, even in cases where the carbide decomposed was of good quality and there was no overheating in the generator. Such discoloration is of little moment for most of the uses to which the sludge may be put. The residue withdrawn from a carbide-to-water generator is usually quite fluid; but when allowed to rest in a suitable pit or tank, it settles down to a semi-solid or pasty mass which contains on a rough average 47 per cent. of water and 53 per cent. of solid matter, the amount of lime present, calculated as calcium oxide, being about 40 per cent. Since 64 parts by weight of pure calcium carbide yield 74 parts of dry calcium hydroxide, it may be said that 1 part of ordinary commercial carbide should yield approximately 1.1 parts of dry residue, or 2.1 parts of a sludge containing 47 per cent. of moisture; and sludge of this character has been stated by Vogel to weigh about 22.5 cwt. per cubic yard.
Experience has shown that those pasty carbide residues can be employed very satisfactorily, and to the best advantage from the maker's point of view, by builders and decorators for the preparation of ordinary mortar or lime-wash. The mortar made from acetylene lime has been found equal in strength and other properties to mortar compounded from fresh slaked lime; while the distemper prepared by diluting the sludge has been used most successfully in all places where a lime-wash is required, e.g., on fruit-trees, on cattle-pens, farm-buildings, factories, and the "offices" of a residence. Many of the village installations abroad sell their sludge to builders for the above-mentioned purposes at such a price that their revenue accounts are materially benefited by the additional income. The sludge is also found serviceable for softening the feed-water of steam boilers by the common liming process; although it has been stated that the material contains certain impurities—notably "fatty matter"—which becomes hydrolysed by the steam, yielding fatty acids that act corrosively upon the boiler-plates. This assertion would appear to require substantiation, but a patent has been taken out for a process of drying the sludge at a temperature of 150 deg. to 200 deg. C. in order to remove the harmful matter by the action of the steam evolved. So purified, it is claimed, the lime becomes fit for treating any hard potable or boiler-feed water. It is very doubtful, however, whether the intrinsic value of acetylene lime is such in comparison with the price of fresh lime that, with whatever object in view, it would bear the cost of any method of artificial drying if obtained from the generators in a pasty state.
When, on the other hand, the residue is naturally dry, or nearly so, it is exactly equal to an equivalent quantity of quick or slaked lime as a dressing for soil. In this last connexion, however, it must be remembered that only certain soils are improved by an addition of lime in any shape, and therefore carbide residues must not be used blindly; but if analysis indicates that a particular plot of ground would derive benefit from an application of lime, acetylene lime is precisely as good as any other description. Naturally a residue containing unspent carbide, or contaminated with tarry matter, is essentially valueless (except as mentioned below); while it must not be forgotten that a solid residue if it is exposed to air, or a pasty residue if not kept under water, will lose many of its useful properties, because it will be partially converted into calcium carbonate or chalk.
Nevertheless, in some respects, the residue from a good acetylene generator is a more valuable material, agriculturally speaking, than pure lime. It contains a certain amount of sulphur, &c., and it therefore somewhat resembles the spent or gas lime of the coal-gas industry. This sulphur, together, no doubt, with the traces of acetylene clinging to it, renders the residue a valuable material for killing the worms and vermin which tend to infest heavily manured and under-cultivated soil. Acetylene lime has been found efficacious in exterminating the "finger-and-toe" of carrots, the "peach-curl" of peach-trees, and in preventing cabbages from being "clubbed." It may be applied to the ground alone, or after admixture with some soil or stable manure. The residue may also be employed, either alone or mixed with some agglomerate, in the construction of garden paths and the like.
If the residues are suitably diluted with water and boiled with (say) twice their original weight of flowers of sulphur, the product consists of a mixture of various compounds of calcium and sulphur, or calcium sulphides—which remain partly in solution and partly in the solid state. This material, used either as a liquid spray or as a moist dressing, has been said to prove a useful garden insecticide and weed-killer.
There are also numerous applications of the acetylene light, each of much value, but involving no new principle which need be noticed. The light is so actinic, or rich in rays acting upon silver salts, that it is peculiarly useful to the photographer, either for portraiture or for his various positive printing operations. Acetylene is very convenient for optical lantern work on the small scale, or where the oxy-hydrogen or oxy-coal-gas light cannot be used. Its intensity and small size make its self-luminous flame preferable on optical grounds to the oil-lamp or the coal-gas mantle; but the illuminating surface is nevertheless too large to give the best results behind such condensers as have been carefully worked to suit a source of light scarcely exceeding the dimensions of a point. For lantern displays on very large screens, or for the projection of a powerful beam of light to great distances in one direction (as in night signalling, &c.), the acetylene blowpipe fed with pure oxygen, or with air containing more than its normal proportion of oxygen, which is discussed in Chapter IX., is specially valuable, more particularly if the ordinary cylinder of lime is replaced by one of magnesia, zirconia, or other highly refractory oxide.
PORTABLE ACETYLENE LAMPS AND PLANT
It will be apparent from what has been said in past chapters that the construction of a satisfactory generator for portable purposes must be a problem of considerable complexity. A fixed acetylene installation tends to work the more smoothly, and the gas evolved therefrom to burn the more pleasantly, the more technically perfect the various subsidiary items of the plant are; that is to say, the more thoroughly the acetylene is purified, dried, and delivered at a strictly constant pressure to the burners and stoves. Moreover, the efficient behaviour of the generator itself will depend more upon the mechanical excellence and solidity of its construction than (with one or two exceptions) upon the precise system to which it belongs. And, lastly, the installation will, broadly speaking, work the better, the larger the holder is in proportion to the demands ever made upon it; while that holder will perform the whole duty of a gasholder more effectually if it belongs to the rising variety than if it is a displacement holder. All these requirements of a good acetylene apparatus have to be sacrificed to a greater or less extent in portable generators; and since the sacrifice becomes more serious as the generator is made smaller and lighter in weight, it may be said in general terms that the smaller a portable (or, indeed, other) acetylene apparatus is, the less complete or permanent satisfaction will it give its user. Again, small portable apparatus are only needed to develop intensities of light insignificant in comparison with those which may easily be won from acetylene on a larger scale; they are therefore fitted with smaller burners, and those burners are not merely small in terms of consumption and illuminating power, but not infrequently are very badly constructed, and are relatively deficient in economy or duty. Thus any comparisons which may be made on lines similar to those adopted in Chapter I., or between unit weights, volumes, or monetary equivalents of calcium carbide, paraffin, candles, and colza oil, become utterly incorrect if the carbide is only decomposed in a small portable generator fitted with an inefficient jet; first, because the latent illuminating power of the acetylene evolved is largely wasted; secondly, because any gas produced over and above that capable of instant combustion must be blown off from a vent-pipe; and thirdly, because the carbide itself tends to be imperfectly decomposed, either through a defect in the construction of the lamp, or through the brief and interrupted requirements of the consumer.