Acetylene, The Principles Of Its Generation And Use
by F. H. Leeds and W. J. Atkinson Butterfield
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of type, from the consumer's point of view may be discussed. The fundamental idea underlying the employment of a non-automatic generator is that the whole of the calcium carbide put into the apparatus shall be decomposed into acetylene as soon after the charge is inserted as is natural in the circumstances; so that after a very brief interval of time the generating chambers shall contain nothing but spent lime and water, and the holder be as full of gas as is ever desirable. In an automatic apparatus, the fundamental idea is that the generating chamber, or one at least of several generating chambers, shall always contain a considerable quantity of undecomposed carbide, and some receptacle always contain a store of water ready to attack that carbide, so that whenever a demand for gas shall arise everything may be ready to meet it. Inasmuch as acetylene is an inflammable gas, it possesses all the properties characteristic of inflammable gases in general; one of which is that it is always liable to take fire in presence of a spark or naked light, and another of which is that it is always liable to become highly explosive in presence of a naked light or spark if, accidentally or otherwise, it becomes mixed with more than a certain proportion of air. On the contrary, in the complete absence of liquid or vaporised water, calcium carbide is almost as inert a body as it is possible to imagine: for it will not take fire, and cannot in any circumstances be made to explode. Hence it may be urged that a non-automatic generator, with its holder always containing a large volume of the actually inflammable and potentially explosive acetylene, must invariably be more dangerous than an automatic apparatus which has less or practically no ready-made gas in it, and which simply contains water in one chamber and unaltered calcium carbide in another. But when the generating vessels and the holder of a non-automatic apparatus are properly designed and constructed, the gas in the latter is acetylene practically free from air, and therefore while being, as acetylene inevitably is, inflammable, is devoid of explosive properties, always assuming, as must be the case in a water-sealed holder, that the temperature of the gas is below 780 deg. C.; and also assuming, as must always be the case in good plant, that the pressure under which the gas is stored remains less than two atmospheres absolute. It is perfectly true that calcium carbide is non-inflammable and non-explosive, that it is absolutely inert and incapable of change; but so comprehensive an assertion only applies to carbide in its original drum, or in some impervious vessel to which moisture and water have no access. Until it is exhausted, an automatic acetylene generator contains carbide in one place and water in another, dependence being put upon some mechanical arrangement to prevent the two substances coming into contact prematurely. Many of the devices adopted by builders of acetylene apparatus for keeping the carbide and water separate, and for mixing them in the requisite quantities when the proper time arrives, are as trustworthy, perhaps, as it is possible for any automatic gear to be; but some are objectionably complicated, and a few are positively inefficient. There are two difficulties which the designer of automatic mechanism has to contend with, and it is doubtful whether he always makes a sufficient allowance for them. The first is that not only must calcium carbide and liquid water be kept out of premature contact, but that moisture, or vapour of water, must not be allowed to reach the carbide; or alternatively, that if water vapour reaches the carbide too soon, the undesired reaction shall not determine overheating, and the liberated gas be not wasted or permitted to become a source of danger. The second difficulty encountered by the designer of automata is so to construct his apparatus that it shall behave well when attended to by completely unskilled labour, that it shall withstand gross neglect and resist positive ill-treatment or mismanagement. If the automatic principle is adopted in any part of an acetylene apparatus it must be adopted throughout, so that as far as possible—and with due knowledge and skill it is completely possible—nothing shall be left dependent upon the memory and common sense of the gasmaker. For instance, it must not be necessary to shut a certain tap, or to manipulate several cocks before opening the carbide vessel to recharge it; it must not be possible for gas to escape backwards out of the holder; and either the carbide-feed gear or the water-supply mechanism (as the case may be) must be automatically locked by the mere act of taking the cover off the carbide store, or of opening the sludge-cock at the bottom. It would be an advantage, even, if the purifiers and other subsidiary items of the plant were treated similarly, arranging them in such fashion that gas should be automatically prevented from escaping out of the rest of the apparatus when any lid was removed. In fact, the general notion of interlocking, which has proved so successful in railway signal-cabins and in carburetted water gas-plant for the prevention of accidents duo to carelessness or overnight, might be copied in principle throughout an acetylene installation whenever the automatic system is employed.

It is no part of the present argument, to allege that automatic generators are, and must always be, inherently dangerous. Automatic devices of a suitable kind may be found in plenty which are remarkably simple and highly trustworthy; but it would be too bold a statement to say that any such arrangement is incapable of failure, especially when put into the hands of a person untrained in the superintendence of machinery. The more reliable a piece of automatic mechanism proves itself to be, the more likely is it to give trouble and inconvenience and utterly to destroy confidence when it does break down; because the better it has behaved in the past, and the longer it has lasted without requiring adjustment, the less likely is it that the attendant will be at hand when failure occurs. By suitable design and by an intelligent employment of safety-valves and blow-off pipes (which will be discussed in their proper place) it is quite easy to avoid the faintest possibility of danger arising from an increase of pressure or an improper accumulation of gas inside the plant or inside the building containing the plant; but every time such a safety-valve or blow-off pipe comes into action a waste of gas occurs, which means a sacrifice of economy, and shows that the generator is not working as it should.

As glass is a fragile and brittle substance, and as it is not capable of bearing large, rapid, and oft-repeated alterations of temperature in perfect safety, it is not a suitable material for the construction of acetylene apparatus or of portions thereof. Hence it follows that a generator must be built of some non-transparent material which prevents the interior being visible when the apparatus is at work. Although it is comparatively easy, by the aid of a lamp placed outside the generator- shed in such a position as to throw its beams of light through a window upon the plant inside, to charge a generator after dark; and although it is possible, without such assistance, by methodical habits and a systematic arrangement of utensils inside the building to charge a generator even in perfect darkness, such an operation is to be deprecated, for it is apt to lead to mistakes, it prevents any slight derangement in the installation from being instantly noticed, and it offers a temptation to the attendant to break rules and to take a naked light with him. On all those grounds, therefore, it is highly desirable that every manipulation connected with a generator shall be effected during the daytime, and that the apparatus-house shall be locked up before nightfall. But owing to the irregular habits engendered by modern life it is often difficult to know, during any given day, how much gas will be required in the ensuing evening; and it therefore becomes necessary always to have, as ready-made acetylene, or as carbide in a proper position for instant decomposition, a patent or latent store of gas more than sufficient in quantity to meet all possible requirements. Now, as already stated, a non-automatic apparatus has its store of material in the form of gas in a holder; and since this is preferably constructed on the rising or telescopic principle, a mere inspection of the height of the bell—on which, if preferred, a scale indicating its contents in cubic feet or in burner-hours may be marked—suffices to show how near the plant is to the point of exhaustion. In many types of automatic apparatus the amount of carbide remaining undecomposed at any moment is quite unknown, or at best can only be deduced by a tedious and inexact calculation; although in some generators, where the store of carbide is subdivided into small quantities, or placed in several different receptacles, an inspection of certain levers or indicators gives an approximate idea as to the capacity of the apparatus for further gas production. In any case the position of a rising holder is the most obvious sign of the degree of exhaustion of a generator; and therefore, to render absolutely impossible a failure of the light during an evening, a non-automatic generator fitted with a rising holder is best.

Since calcium carbide is a solid body having a specific gravity of 2.2, water being unity, and since 1 cubic foot of water weighs 62.4 lb., in round numbers 137 lb. of compact carbide only occupy 1 cubic foot of space. Again, since acetylene is a gas having a specific gravity of 0.91, air being unity, and since the specific gravity of air, water being unity, is 0.0013, the specific gravity of acetylene, water being unity, is roughly O.00116. Hence 1 cubic foot of acetylene weighs roughly 0.07 lb. Furthermore, since 1 lb. of good carbide evolves 5 cubic feet of gas on decomposition with water, acetylene stored at atmospheric pressure occupies roundly 680 times as much space as the carbide from which it has been evolved. This figure by no means represents the actual state of affairs in a generator, because, as was explained in the previous chapter, a carbide vessel cannot be filled completely with solid; and, indeed, were it so "filled," in ordinary language, much of its space would be still occupied with air. Nevertheless it is incontrovertible that an acetylene plant calculated to supply so many burners for so long a period of time must be very much larger if it is constructed on the non-automatic principle, when the carbide is decomposed all at once, than if the automatic system is adopted, when the solid remains unattacked until a corresponding quantity of gas is required for combustion. Clearly it is the storage part of a non-automatic plant alone which must be so much larger; the actual decomposing chambers may be of the same size or even smaller, according to the system of generation to which the apparatus belongs. In practice this extra size of the non-automatic plant causes it to exhibit two disadvantages in comparison with automatic apparatus, disadvantages which are less serious than they appear, or than they may easily be represented to be. In the first place, the non- automatic generator requires more space for its erection. If acetylene were an illuminating agent suitable for adoption by dwellers in city or suburb, where the back premises and open-air part of the messuage are reduced to minute proportions or are even non-existent, this objection might well be fatal. But acetylene is for the inhabitant of a country village or the occupier of an isolated country house; and he has usually plenty of space behind his residence which he can readily spare. In the second place, the extra size of the non-automatic apparatus makes it more expensive to construct and more costly to instal. It is more cosily to construct and purchase because of its holder, which must be well built on a firm foundation and accurately balanced; it is more costly to instal because a situation must be found for the erection of the holder, and the apparatus-house may have to be made large enough to contain the holder as well as the generator itself. As regards the last point, it may be said at once that there is no necessity to place the holder under cover: it may stand out of doors, as coal-gas holders do in England, for the seal of the tank can easily be rendered frost-proof, and the gas itself is not affected by changes of atmospheric temperature beyond altering somewhat in volume. In respect of the other objections, it must be remembered that the extra expense is one of capital outlay alone, and therefore only increases the cost of the light by an inappreciable amount, representing interest and depreciation charges on the additional capital expenditure. The increased cost of a year's lighting due to these charges will amount to only 10 or 15 per cent, on the additional capital sunk. The extra capital sunk does not in any way increase the maintenance charges; and if, by having a large holder, additional security and trustworthiness are obtained, or if the holder leads to a definite, albeit illusive, sense of extra security and trustworthiness, the additional expenditure may well be permissible or even advantageous.

The argument is sometimes advanced that inasmuch as for the same, or a smaller, capital outlay as is required to instal a non-automatic apparatus large enough to supply at one charging the maximum amount of light and heat that can ever be needed on the longest winter's night, an automatic plant adequate to make gas for two or three evenings can be laid down, the latter must be preferable, because the attendant, in the latter case, will only need to enter the generator-house two or three times a week. Such an argument is defective because it ignores the influence of habit upon the human being. A watch which must be wound every day, or a clock which must be wound every week, on a certain day of the week, is seldom permitted to run down; but a watch requiring to be re-wound every other day, or a fourteen-day clock (used as such), would rarely be kept going. Similarly, an acetylene generator might be charged once a week or once a day without likelihood of being forgotten; but the operation of charging at irregular intervals would certainly prove a nuisance. With a non-automatic apparatus containing all its gas in the holder, the attendant would note the position of the bell each morning, and would introduce sufficient carbide to fill the holder full, or partly full, as the case might be; with an automatic apparatus he would be tempted to trust that the carbide holders still contained sufficient material to last another night.

The automatic system of generating acetylene has undoubtedly one advantage in those climates where frost tends to occur frequently, but only to prevail for a short period. As the apparatus is in operation during the evening hours, the heat evolved will, or can be made to, suffice to protect the apparatus from freezing until the danger has passed; whereas if the gas is generated of a morning in a non-automatic apparatus the temperature of the plant may fall to that of the atmosphere before evening, and some portion may freeze unless special precautions are taken to protect it.

It was shown in Chapter II that overheating is one of the chief troubles to be guarded against in acetylene generators, and that the temperature attained is a function of the speed at which generation proceeds. Seeing that in an automatic apparatus the rate of decomposition depends on the rate at which gas is being burnt, while in a non-automatic generator it is, or may be, under no control, the critic may urge that the reaction must take place more slowly and regularly, and the maximum temperature therefore be lower, when the plant works automatically. This may be true if the non-automatic generator is unskilfully designed or improperly manipulated; but it is quite feasible to arrange an apparatus, especially one of the carbide-to-water or of the flooded-compartment type, in such fashion that overheating to an objectionable extent is rendered wholly impossible. In a non-automatic apparatus the holder is nothing but a holder and may be placed wherever convenient, even at a distance from the generating plant; in an automatic apparatus the holder, or a small similarly constructed holder placed before the main storage vessel, has to act as a water-supply governor, as the releasing gear for certain carbide-food mechanism, or indeed as the motive power of such mechanism; and accordingly it must be close to the water or carbide store, and more or less intimately connected by means of levers, or the like, with the receptacle in which decomposition occurs. Sometimes the holder surrounds, or is otherwise an integral part of, the decomposing chamber, the whole apparatus being made self-contained or a single structure with the object of gaining compactness. But it is evident that such methods of construction render additionally awkward, or even hazardous, any repair or petty operation to the generating portion of the plant; while the more completely the holder is isolated from the decomposing vessels the more easily can they be cleaned, recharged, or mended, without blowing off the stored gas and without interfering with the action of any burners that may be alight at the time. Owing to the ingenuity of inventors, and the experience they have acquired in the construction of automatic acetylene apparatus during the years that the gas has been in actual employment, it is going too far boldly to assert that non-automatic generators are invariably to be preferred before their rivals. Still in view of the nature of the labour which is likely to be bestowed on any domestic plant, of the difficulty in having repairs or adjustments done quickly in outlying country districts, and of the inconvenience, if not risk, attending upon any failure of the apparatus, the greater capital outlay, and the larger space required by non-automatic generators are in most instances less important than the economy in space and prime cost characteristic of automatic machines when the defects of each are weighed fairly in the balance. Indeed, prolonged experience tends to show that a selection between non-automatic and automatic apparatus may frequently be made on the basis of capacity. A small plant is undoubtedly much more convenient if automatic; a very large plant, such as that intended for a public supply, is certainly better if non-automatic, but between these two extremes choice may be exercised according to local conditions.

CONTROL OF THE CHEMICAL REACTION.—Coming now to study the principles underlying the construction of an acetylene generator more closely it will be seen that as acetylene is produced by bringing calcium carbide into contact with water, the chemical reaction may be started either by adding the carbide to the water, or by adding the water to the carbide. Similarly, at least from the theoretical aspect, the reaction, may be caused to stop by ceasing to add carbide to water, or by ceasing to add water to carbide. Apparently if water is added by degrees to carbide, until the carbide is exhausted, the carbide must always be in excess; and manifestly, if carbide is added in small portions to water, the water must always be in excess, which, as was argued in Chapter II., is emphatically the more desirable position of affairs. But it in quite simple to have carbide present in large excess of the water introduced when the whole generator is contemplated, and yet to have the water always in chemical excess in the desired manner; because to realise the advantages of having water in excess, it is only necessary to subdivide the total charge of carbide into a number of separate charges which are each so small that more than sufficient water to decompose and flood one of them is permitted to enter every time the feed mechanism comes into play, or (in a non-automatic apparatus) every time the water-cock is opened; so arranging the charges that each one is protected from the water till its predecessor, or its predecessor, have been wholly decomposed. Thus it is possible to regard either the carbide or the water as the substance which has to be brought into contact with the other in specified quantity. It is perhaps permissible to repeat that in the construction of an automatic generator there is no advantage to be gained from regulating the supply of both carbide and water, because just as the mutual decomposition will begin immediately any quantity of the one meets any quantity of the other, so the reaction will cease (except in one case owing to "after-generation") directly the whole of that material which is not in chemical excess has been consumed-quite independently of the amount of the other material left unattacked. Being a liquid, and possessing as such no definite shape or form of its own irrespective of the vessel in which it is held, water is by far the more convenient of the two substances to move about or to deliver in predetermined volume to the decomposing chamber. A supply of water can be started instantaneously or cut oil as promptly by the movement of a cock or valve of the usual description; or it may be allowed to run down a depending pipe in obedience to the law of gravitation, and stopped from running down such a pipe by opposing to its passage a gas pressure superior to that gravitational force. In any one of several obvious ways the supply of water to a mass of carbide may be controlled with absolute certainty, and therefore it should apparently follow that the make of acetylene should be under perfect control by controlling the water current. On the other hand, unless made up into balls or cartridges of some symmetrical form, calcium carbide exists in angular masses of highly irregular shape and size. Its lumps alter in shape and size directly liquid water or moisture reaches them; a loose more or loss gritty powder, or a damp cohesive mud, being produced which is well calculated to choke any narrow aperture or to jam any moving valve. It is more difficult, therefore, by mechanical agency to add a supply of carbide to a mass of water than to introduce a supply of water to a stationary mass of carbide; and far more difficult still to bring the supply of carbide under perfect control with the certainty that the movement shall begin and stop immediately the proper time arrives.

But assuming the mechanical difficulties to be satisfactorily overcome, the plan of adding carbide to a stationary mass of water has several chemical advantages, first, because, however the generator be constructed, water will be in excess throughout the whole time of gas production; and secondly, because the evolution of acetylene will actually cease completely at the moment when the supply of carbide is interrupted. There is, however, one particular type of generator in which as a matter of fact the carbide is the moving constituent, viz., the "dipping" apparatus (cf. infra), to which these remarks do not apply; but this machine, as will be seen directly, is, illogically perhaps, but for certain good reasons, classed among the water-to-carbide apparatus. All the mechanical advantages are in favour, as just indicated, of making water the moving substance; and accordingly, when classified in the present manner, a great majority of the generators now on the markets are termed water-to-carbide apparatus. Their disadvantages are twofold, though these may be avoided or circumvented: in all types save one the carbide is in excess at the immediate place and time of decomposition; and in all types without exception the carbide in the whole of the generator is in excess, so that the phenomenon of "after- generation" occurs with more or less severity. As explained in the last chapter, after-generation is the secondary production of acetylene which takes place more or less slowly after the primary reaction is finished, proceeding either between calcium hydroxide, merely damp lime, or damp gas and calcium carbide, with an evolution of more acetylene. As it is possible, and indeed usual, to fit a holder of some capacity even to an automatic generator, the simple fact that more acetylene is liberated after the main reaction is over does not matter, for the gas can be safely stored without waste and entirely without trouble or danger. The real objection to after-generation is the difficulty of controlling the temperature and of dissipating the heat with which the reaction is accompanied. It will be evident that the balance of advantage, weighing mechanical simplicity against chemical superiority, is somewhat even between carbide-to-water and water-to-carbide generators of the proper type; but the balance inclines towards the former distinctly in the ease of non-automatic apparatus, and points rather to the latter when automatism is desired. In the early days of the industry it would have been impossible to speak so favourably of automatic carbide-to-water generators, for they were at first constructed with absurdly complicated and unreliable mechanism; but now various carbide-feed gears have been devised which seem to be trustworthy even when carbide not in cartridge form is employed.

NON-AUTOMATIC CARBIDE-TO-WATER GENERATORS.—There is little to be said in the present place about the principles underlying the construction of non-automatic generators. Such apparatus may either be of the carbide-to- water or the water-to-carbide type. In the former, lumps of carbide are dropped by hand down a vertical or sloping pipe or shoot, which opens at its lower end below the water-level of the generating chamber, and which is fitted below its mouth with a deflector to prevent the carbide from lodging immediately underneath that mouth. The carbide falls through the water which stands in the shoot itself almost instantaneously, but during its momentary descent a small quantity of gas is evolved, which produces an unpleasant odour unless a ventilating hood is fixed above the upper end of the tube. As the ratio of cubical contents to superficial area of a lump is greater as the lump itself is larger, and as only the outer surface of the lump can be attacked by the water in the shoot during its descent, carbide for a hand-fed carbide-to-water generator should be in fairly large masses—granulated material being wholly unsuitable—and this quite apart from the fact that large carbide is superior to small in gas-making capacity, inasmuch as it has not suffered the inevitable slight deterioration while being crushed and graded to size. If carbide is dropped too rapidly into such a generator which is not provided with a false bottom or grid for the lumps to rest upon, the solid is apt to descend among a mass of thick lime sludge produced at a former operation, which lies at the bottom of the decomposing chamber; and here it may be protected from the cooling action of fresh water to such an extent that its surface is baked or coated with a hard layer of lime, while overheating to a degree far exceeding the boiling-point of water may occur locally. When, however, it falls upon a grid placed some distance above the bottom of the water vessel, the various convection currents set up as parts of the liquid become warm, and the mechanical agitations produced by the upward current of gas rinse the spent lime from the carbide, and entirely prevent overheating, unless the lumps are excessively large in size. If the carbide charged into a hand-fed generator is in very large lumps there is always a possibility that overheating may occur in the centre of the masses, due to the baking of the exterior, even if the generator is fitted with a reaction grid. Manifestly, when carbide in lumps of reasonable size is dropped into excess of water which is not merely a thick viscid cream of lime, the temperature cannot possibly exceed the boiling-point—i.e., 100 deg. C.—provided always the natural convection currents of the water are properly made use of.

The defect which is, or rather which may be, characteristic of a hand-fed carbide-to-water generator is a deficiency of gas yield due to solubility. At atmospheric temperatures and pressure 10 volumes of water dissolve 11 volumes of acetylene, and were the whole of the water in a large generator run to waste often, a sensible loss of gas would ensue. If the carbide falls nearly to the bottom of the water column, the rising gas is forced to bubble through practically the whole of the liquid, so that every opportunity is given it to dissolve in the manner indicated till the liquid is completely saturated. The loss, however, is not nearly so serious as is sometimes alleged, because (1) the water becomes heated and so loses much of its solvent power; and (2) the generator is worked intermittently, with sufficiently long intervals to allow the spent lime to settle into a thick cream, and only that thick cream is run off, which represents but a small proportion of the total water present. Moreover, a hand-fed carbide-to-water generator will work satisfactorily with only half a gallon [Footnote: The United States National Board of Fire Underwriters stipulates for the presence of 1 (American) gallon of water for every 1 lb. of carbide before such an apparatus is "permitted." This quantity of liquid might retain nearly 4 per cent. of the total acetylene evolved. Even this is an exaggeration; for neither her, nor in the corresponding figure given in the text, is any allowance made for the diminution in solvent power of the water as it becomes heated by the reaction.] of liquid present for every 1 lb. of carbide decomposed, and were all this water run off and a fresh quantity admitted before each fresh introduction of carbide, the loss of acetylene by dissolution could not exceed 2 per cent. of the total make, assuming the carbide to be capable of yielding 5 cubic feet of gas per lb. Admitting, however, that some loss of gas does occur in this manner, the defect is partly, if not wholly, neutralised by the concomitant advantages of the system: (1) granted that the generator is efficiently constructed, decomposition of the carbide is absolutely complete, so that no loss of gas occurs in this fashion; (2) the gas is evolved at a low temperature, so that it is unaccompanied, by products of polymerisation, which may block the leading pipes and must reduce the illuminating power; (3) the acetylene is not mixed with air (as always happens at the first charging of a water-to- carbide apparatus), which also lowers the illuminating power; and (4) the gas is freed from two of its three chief impurities, viz., ammonia and sulphuretted hydrogen, in the generating chamber itself. To prevent the loss of acetylene by dissolution, carbide-to-water generators are occasionally fitted with a reaction grid placed only just below the water-level, so that the acetylene has no more than 1 inch or so of liquid to bubble through. The principle is wrong, because hot water being lighter than cold, the upper layers may be raised to the boiling-point, and even converted into steam, while the bulk of the liquid still remains cold; and if the water actually surrounding the carbide is changed into vapour, nearly all control over the temperature attending the reaction is lost.

The hand-fed carbide-to-water generator is very simple and, as already indicated, has proved itself perhaps the best type of all for the construction of very large installations; but the very simplicity of the generator has caused it more than once to be built in a manner that has not given entire satisfaction. As shown at L in Fig. 6, p. 84, the generator essentially consists of a closed cylindrical vessel communicating at its top with a separate rising holder. At one side as drawn, or disposed concentrically if so preferred, is an open-mouthed pipe or shoot (American "shute") having its lower open extremity below the water-level. Into this shoot are dropped by hand or shovel lumps of carbide, which fall into the water and there suffer decomposition. As the bottom of the shoot is covered with water, which, owing to the small effective gas pressure in the generator given by the holder, stands a few inches higher in the shoot than in the generator, gas cannot escape from the shoot; because before it could do so the water in the generator would have to fall below the level of the point a, being either driven out through the shoot or otherwise. Since the point b of the shoot extends further into the generator than a, the carbide drops centrally, and as the bubbles of gas rise vertically, they have no opportunity of ascending into the shoot. In practice, the generator is fitted with a conical bottom for the collection of the lime sludge and with a cock or other aperture at the apex of the cone for the removal of the waste product. As it is not desirable that the carbide should be allowed to fall directly from the shoot into the thicker portion of the sludge within the conical part of the generator, one or more grids is usually placed in the apparatus as shown by the dotted lines in the sketch. It does not seem that there is any particular reason for the employment of more than one grid, provided the size of the carbide decomposed is suited to the generator, and provided the mesh of the grid is suited to the size of the carbide. A great improvement, however, is made if the grid is carried on a horizontal spindle in such a way that it can be rocked periodically in order to assist in freeing the lumps of carbide from the adhering particles of lime. As an alternative to the movable grid, or even as an adjunct thereto, an agitator scraping the conical sides of the generator may be fitted which also assists in ensuring a reasonably complete absence of undecomposed carbide from the sludge drawn off at intervals. A further point deserves attention. If constructed in the ideal manner shown in Fig. 6 removal of some of the sludge in the generator would cause the level of the liquid to descend and, by carelessness, the level might fall below the point a at the base of the shoot. In these circumstances, if gas were unable to return from the holder, a pressure below that of the atmosphere would be established in the gas space of the generator and air would be drawn in through the shoot. This air might well prove a source of danger when generation was started again. Any one of three plans may be adopted to prevent the introduction of air. A free path may be left on the gas-main passing from the generator to the holder so that gas may be free to return and so to maintain the usual positive pressure in the decomposing vessel; the sludge may be withdrawn into some vessel so small in capacity that the shoot cannot accidentally become unsealed; or the waterspace of the generator may be connected with a water-tank containing a ball-valve attached to a constant service of water be that liquid runs in as quickly as sludge is removed, and the level remains always at the same height. The first plan is only a palliative and has two defects. In the first place, the omission of any non-return valve between, the generator and the next item in the train of apparatus is objectionable of itself; in the second place, should a very careless attendant withdraw too much liquid, the shoot might become unsealed and the whole contents of the holder be passed into the air of the building containing the apparatus through the open mouth of the shoot. The second plan is perfectly sound, but has the practical defect of increasing the labour of cleaning the generator. The third plan is obviously the best. It can indeed be adopted where no real constant service of water is at hand by connecting the generator to a water reservoir of relatively large size and by making the latter of comparatively large transverse area, in proportion to its depth; so that the escape of even a largo volume of water from the reservoir may not involve a large reduction in the level at which it stands there.

The dust that always clings to lumps of carbide naturally decomposes with extreme rapidity when the material is thrown into the shoot of a carbide- to-water generator, and the sudden evolution of gas so produced has on more than one occasion seriously alarmed the attendant on the plant. Moreover, to a trifling extent the actual superficial layers of the carbide suffer attack before the lumps reach the true interior of the generator, and a small loss of gas thereby occurs through the open mouth of the shoot. To remove these objections to the hand-fed generator it has become a common practice in large installations to cause the lower end of the shoot to dip under the level of some oil contained in an appropriate receptacle, the carbide falling into a basket carried upon a horizontal spindle. The basket and its support are so arranged that when a suitable charge of carbide has been dropped into it, a partial rotation of an external hand-wheel lifts the basket and carbide out of the oil into an air-tight portion of the generator where the surplus oil can drain away from the lumps. A further rotation of the hand-wheel then tips the basket over a partition inside the apparatus, allowing the carbide to fall into the actual decomposing chamber. This method of using oil has the advantage of making the evolution of acetylene on a large scale appear to proceed more quietly than usual, and also of removing the dust from the carbide before it reaches the water of the generator. The oil itself obviously does not enter the decomposing chamber to any appreciable extent and therefore does not contaminate the final sludge. The whole process accordingly lies to be favourably distinguished from those other methods of employing oil in generators or in the treatment of carbide which are referred to elsewhere in this book.

NON-AUTOMATIC WATER-TO-CARBIDE GENERATORS.—The only principle underlying the satisfactory design of a non-automatic water-to-carbide generator is to ensure the presence of water in excess at the spot where decomposition is taking place. This may be effected by employing what is known as the "flooded-compartment" system of construction, i.e., by subdividing the total carbide charge into numerous compartments arranged either vertically or horizontally, and admitting the water in interrupted quantities, each more than sufficient thoroughly to decompose and saturate the contents of one compartment, rather than in a slow, steady stream. It would be quite easy to manage this without adopting any mechanism of a moving kind, for the water might be stored in a tank kept full by means of a ball-valve, and admitted to an intermediate reservoir in a slow, continuous current, the reservoir being fitted with an inverted syphon, on the "Tantalus-cup" principle, so that it should first fill itself up, and then suddenly empty into the pipe leading to the carbide container. Without this refinement, however, a water-to-carbide generator, with subdivided charge, behaves satisfactorily as long as each separate charge of carbide is so small that the heat evolved on its decomposition can be conducted away from the solid through the water- jacketed walls of the vessel, or as the latent heat of steam, with sufficient rapidity. Still it must be remembered that a water-to-carbide generator, with subdivided charge, does not belong to the flooded- compartment type if the water runs in slowly and continuously: it is then simply a "contact" apparatus, and may or may not exhibit overheating, as well as the inevitable after-generation. All generators of the water-to- carbide type, too, must yield a gas containing some air in the earlier portions of their make, because the carbide containers can only be filled one-third or one-half full of solid. Although the proportion of air so passed into the holder may be, and usually is, far too small in amount to render the gas explosive or dangerous in the least degree, it may well be sufficient to reduce the illuminating power appreciably until it is swept out of the service by the purer gas subsequently generated. Moreover, all water-to-carbide generators are liable, as just mentioned, to produce sufficient overheating to lower the illuminating power of the gas whenever they are wilfully driven too fast, or when they are reputed by their makers to be of a higher productive capacity than they actually should be; and all water-to-carbide generators, excepting those where the carbide is thoroughly soaked in water at some period of their operation, are liable to waste gas by imperfect decomposition.

DEVICES TO SECURE AUTOMATIC ACTION,—The devices which are commonly employed to render a generator automatic in action, that is to say, to control the supply of one of the two substances required in the intermittent evolution of gas, may be divided into two broad classes: (A) those dependent upon the position of a rising-holder bell, and (B) those dependent upon the gas pressure inside the apparatus. As the bell of a rising holder descends in proportion as its gaseous contents are exhausted, it may (A^1) be fitted with some laterally projecting pin which, arrived at a certain position, actuates a series of rods or levers, and either opens a cock on the water-supply pipe or releases a mechanical carbide-feed gear, the said cock being closed again or the feed-gear thrown out of action when the pin, rising with the bell, once more passes a certain position, this time in its upward path. Secondly (A^2), the bell may be made to carry a perforated receptacle containing carbide, which is dipped into the water of the holder tank each time the bell falls, and is lifted out of the water when it rises again. Thirdly (A^3), by fitting inside the upper part of the bell a false interior, conical in shape, the descent of the bell may cause the level of the water in the holder tank to rise until it is above some lateral aperture through which the liquid may escape into a carbide container placed elsewhere. These three methods are represented in the annexed diagram (Fig. 1). In Al the water-levels in the tank and bell remain always at l, being higher in the tank than in the bell by a distance corresponding with the pressure produced by the bell itself. As the bell falls a pin X moves the lever attached to the cock on the water- pipe, and starts, or shuts off, a current passing from a store-tank or reservoir to a decomposing vessel full of carbide. It is also possible to make X work some releasing gear which permits carbide to fall into water—details of this arrangement are given later on. In A^1 the water in the tank serves as a holder seal only, a separate quantity being employed for the purposes of the chemical reaction. This arrangement has the advantage that the holder water lasts indefinitely, except for evaporation in hot weather, and therefore it may be prevented from freezing by dissolving in it some suitable saline body, or by mixing with it some suitable liquid which lowers its point of solidification. It will be observed, too, that in A^1 the pin X, which derives its motive power from the surplus weight of the falling bell, has always precisely the same amount of work to do, viz., to overcome the friction of the plug of the water-cock in its barrel. Hence at all times the pressure obtaining in the service-pipe is uniform, except for a slight jerk momentarily given each time the cock is opened or closed. When X actuates a carbide-feed arrangement, the work it does may or may not vary on different occasions, as will appear hereafter. In A^2 the bell itself carries a perforated basket of carbide, which is submerged in the water when the bell falls, and lifted out again when it rises. As the carbide is thus wetted from below, the lower portion of the mass soon becomes a layer of damp slaked lime, for although the basket is raised completely above the water-level, much liquid adheres to the spent carbide by capillary attraction. Hence, even when the basket is out of the water, acetylene is being produced, and it is produced in circumstances which prevent any control over the temperature attained. The water clinging to the lower part of the basket is vaporised by the hot, half-spent carbide, and the steam attacks the upper part, so that polymerisation of the gas and baking of the carbide are inevitable. In the second place, the pressure in the service-pipe attached to A^2 depends as before upon the net weight of the holder bell; but here that net weight is made up of the weight of the bell itself, that of the basket, and that of the carbide it contains. Since the carbide is being gradually converted into damp slaked lime, it increases in weight to an indeterminate extent as the generator in exhausted; but since, on the other hand, some lime may be washed out of the basket each time it is submerged, and some of the smaller fragments of carbide may fall through the perforations, the basket tends to decrease in weight as the generator is exhausted. Thus it happens in A^2 that the combined weight of bell plus basket plus contents is wholly indefinite, and the pressure in the service becomes so irregular that a separate governor must be added to the installation before the burners can be expected to behave properly. In the third place, the water in the tank serves both for generation and for decomposition, and this involves the employment of some arrangement to keep its level fairly constant lest the bell should become unsealed, while protection from frost by saline or liquid additions is impossible. A^2 is known popularly as a "dipping" generator, and it will be seen to be defective mechanically and bad chemically. In both A^1 and A^2 the bell is constructed of thin sheet- metal, and it is cylindrical in shape; the mass of metal in it is therefore negligible in comparison with the mass of water in the tank, and so the level of the liquid is sensibly the same whether the bell be high or low. In A^3 the interior of the bell is fitted with a circular plate which cuts off its upper corners and leaves a circumferential space S triangular in vertical section. This space is always full of air, or air and water, and has to be deducted from the available storage capacity of the bell. Supposing the bell transparent, and viewing it from above, its effective clear or internal diameter will be observed to be smaller towards the top than near the bottom; or since the space S is closed both against the water and against the gas, the walls of the bell may be said to be thicker near its top. Thus it happens that as the bell descends into the water past the lower angle of S, it begins to require more space for itself in the tank, and so it displaces the water until the levels rise. When high, as shown in the sketch marked A^3(a), the water-level is at l, below the mouth of a pipe P; but when low, as in A^3(b), the water is raised to the point l', which is above P. Water therefore flows into P, whence it reaches the carbide in an attached decomposing chamber. Here also the water in the tank is used for decomposition as well as for sealing purposes, and its normal level must be maintained exactly at l, lest the mouth of P should not be covered whenever the bell falls.

The devices employed to render a generator automatic which depend upon pressure (B) are of three main varieties: (B^1) the water-level in the decomposing chamber may be depressed by the pressure therein until its surface falls below a stationary mass of carbide; (B^2) the level in a water-store tank may be depressed until it falls below the mouth of a pipe leading to the carbide vessel; (B^3) the current of water passing down a pipe to the decomposing chamber may be interrupted by the action of a pressure superior to the force of gravitation. These arrangements are indicated roughly in Fig. 2. In B^1, D is a hollow cylinder closed at all points except at the cock G and the hole E, which are always below the level of the water in the annulus F, the latter being open to the air at its top. D is rigidly fastened to the outer vessel F so that it cannot move vertically, and the carbide cage is rigidly fastened to D. Normally the water-levels are at l, and the liquid has access to the carbide through perforations in the basket. Acetylene is thus produced; but if G is shut, the gas is unable to escape, and so it presses downwards upon the water until the liquid falls in D to the dotted line l", rising in F to the dotted line l'. The carbide is then out of water, and except for after-generation, evolution of gas ceases. On opening G more or less fully, the water more or less quickly reaches its original position at l, and acetylene is again produced. Manifestly this arrangement is identical with that of A^2 as regards the periodical immersion of the carbide holder in the liquid; but it is even worse than the former mechanically because there is no rising holder in B^1, and the pressure in the service is never constant. B^2 represents the water store of an unshown generator which works by pressure. It consists of a vessel divided vertically by means of a partition having a submerged hole N. One-half, H, is cloned against the atmosphere, but communicates with the gas space of the generator through L; the other half, K, is open to the air. M is a pipe leading water to the carbide. When gas is being burnt as fast as, or faster than, it is being evolved, the pressure in the generator is small, the level of the water stands at l, and the mouth of M is below it. When the pressure rises by cessation of consumption, that pressure acts through L upon the water in H, driving it down in H and up in K till it takes the positions l", and l', the mouth of M being then above the surface. It should be observed that in the diagrams B^1 and B^3, the amount of pressure, and the consequent alteration in level, is grossly exaggerated to gain clearness; one inch or less in both cases may be sufficient to start or retard evolution of acetylene. Fig. B^3 is somewhat ideal, but indicates the principle of opposing gas pressure to a supply of water depending upon gravitation; a method often adopted in the construction of portable acetylene apparatus. The arrangement consists of an upper tank containing water open to the air, and a lower vessel holding carbide closed everywhere except at the pipe P, which leads to the burners, and at the pipe S, which introduces water from the store-tank. If the cock at T is closed, pressure begins to rise in the carbide holder until it is sufficient to counterbalance the weight of the column of water in the pipe S, when a further supply is prevented until the pressure sinks again. This idea is simply an application of the displacement-holder principle, and as such is defective (except for vehicular lamps) by reason of lack of uniformity in pressure.

DISPLACEMENT GASHOLDERS.—An excursion may here be made for the purpose of studying the action of a displacement holder, which in its most elementary form is shown at C. It consists of an upright vessel open at the top, and divided horizontally into two equal portions by a partition, through which a pipe descends to the bottom of the lower half. At the top of the closed lower compartment a tube is fixed, by means of which gas can be introduced below the partition. While the cock is open to the air, water is poured in at the open top till the lower compartment is completely full, and the level of the liquid is at l. If now, gas is driven in through the side tube, the water is forced downwards in the lower half, up through the depending pipe till it begins to fill the upper half of the holder, and finally the upper half is full of water and the lower half of gas an shown by the levels l' and l". But the force necessary to introduce gas into such an apparatus, which conversely is equal to the force with which the apparatus strives to expel its gaseous contents, measured in inches of water, is the distance at any moment between the levels l' and l"; and as these are always varying, the effective pressure needed to fill the apparatus, or the effective pressure given by the apparatus, may range from zero to a few inches less than the total height of the whole holder. A displacement holder, accordingly, may be used either to store a varying quantity of gas, or to give a steady pressure just above or just below a certain desired figure; but it will not serve both purposes. If it is employed as a holder, it in useless as a governor or pressure regulator; if it is used as a pressure regulator, it can only hold a certain fixed volume of gas. The rising holder, which is shown at A^1 in Fig. 1 (neglecting the pin X, &c.) serves both purposes simultaneously; whether nearly full or nearly empty, it gives a constant pressure—a pressure solely dependent upon its effective weight, which may be increased by loading its crown or decreased by supporting it on counterpoises to any extent that may be required. As the bell of a rising holder moves, it must be provided with suitable guides to keep its path vertical; these guides being arranged symmetrically around its circumference and carried by the tank walls. A fixed control rod attached to the tank over which a tube fastened to the bell slides telescope-fashion is sometimes adopted; but such an arrangement is in many respects less admirable than the former.

Two other devices intended to give automatic working, which are scarcely capable of classification among their peers, may be diagrammatically shown in Fig. 3. The first of these (D) depends upon the movements of a flexible diaphragm. A vessel (a) of any convenient size and shape is divided into two portions by a thin sheet of metal, leather, caoutchouc, or the like. At its centre the diaphragm is attached by some air-tight joint to the rod c, which, held in position by suitable guides, is free to move longitudinally in sympathy with the diaphragm, and is connected at its lower extremity with a water-supply cock or a carbide-feed gear. The tube e opens at its base into the gas space of the generator, so that the pressure below the diaphragm in a is the same as that elsewhere in the apparatus, while the pressure in a above the diaphragm is that of the atmosphere. Being flexible and but slightly stretched, the diaphragm is normally depressed by the weight of c until it occupies the position b; but if the pressure in the generator (i.e., in e) rises, it lifts the diaphragm to somewhat about the position b'—the extent of movement being, as usual, exaggerated in the sketch. The movement of the diaphragm is accompanied by a movement of the rod c, which can be employed in any desirable way. In E the bell of a rising holder of the ordinary typo is provided with a horizontal striker which, when the bell descends, presses against the top of a bag g made of any flexible material, such as india-rubber, and previously filled with water. Liquid is thus ejected, and may be caused to act upon calcium carbide in some adjacent vessel. The sketch is given because such a method of obtaining an intermittent water-supply has at one time been seriously proposed; but it is clearly one which cannot be recommended.

ACTION OF WATER-TO-CARBIDE GENERATORS.—Having by one or other of the means described obtained a supply of water intermittent in character, it remains to be considered how that supply may be made to approach the carbide in the generator. Actual acetylene apparatus are so various in kind, and merge from one type to another by such small differences, that it is somewhat difficult to classify them in a simple and intelligible fashion. However, it may be said that water-to-carbide generators, i.e., such as employ water as the moving material, may be divided into four categories: (F^1) water is allowed to fall as single drops or as a fine stream upon a mass of carbide—this being the "drip" generator; (F^2) a mass of water is made to rise round and then recede from a stationary vessel containing carbide—this being essentially identical in all respects save the mechanical one with the "dip" or "dipping" generator shown in A^2, Fig. 1; (F^3) a supply of water is permitted to rise round, or to flow upon, a stationary mass of carbide without ever receding from the position it has once assumed—this being the "contact" generator; and (F^4) a supply of water is admitted to a subdivided charge of carbide in such proportion that each quantity admitted is in chemical excess of the carbide it attacks. With the exception of F^2, which has already been illustrated as A^2 Fig. 1, or as B^1 in Fig. 2, these methods of decomposing carbide are represented in Figs. 4 and 5. It will be observed that whereas in both F^1 and F^3 the liberated acetylene passes off at the top of the apparatus, or rather from the top of the non-subdivided charge of carbide, in F^1 the water enters at the top, and in F^3 it enters at the bottom. Thus it happens that the mixture of acetylene and steam, which is produced at the spot where the primary chemical reaction is taking place, has to travel through the entire mass of carbide present in a generator belonging to type F^3, while in F^1 the damp gas flows directly to the exit pipe without having to penetrate the lumps of solid. Both F^1 and F^3 exhibit after-generation caused by a reaction between the liquid water mechanically clinging to the mass of spent lime and the excess of carbide to an approximately equal extent; but for the reason just mentioned, after-generation due to a reaction between the vaporised water accompanying the acetylene first evolved and the excess of carbide is more noticeable in F^3 than in F^1; and it is precisely this latter description of after-generation which leads to overheating of the most ungovernable kind. Naturally both F^1 and F^3 can be fitted with water jackets, as is indicated by the dotted lines in the second sketch; but unless the generating chamber in quite small and the evolution of gas quite slow, the cooling action of the jacket will not prove sufficient. As the water in F^1 and F^3 is not capable of backward motion, the decomposing chambers cannot be employed as displacement holders, as is the case in the dipping generator pictured at B^1, Fig. 2. They must be coupled, accordingly, to a separate holder of the displacement or, preferably, of the rising type; and, in order that the gas evolved by after-generation may not be wasted, the automatic mechanism must cut off the supply of water to the generator by the time that holder is two-thirds or three-quarters full.

The diagrams G, H, and K in Figs. 4 and 5 represent three different methods of constructing a generator which belongs either to the contact type (F^3) if the supply of water is essentially continuous, i.e., if less is admitted at each movement of the feeding mechanism than is sufficient to submerge the carbide in each receptacle; or to the flooded- compartment type (F') if the water enters in large quantities at a time. In H the main carbide vessel is arranged horizontally, or nearly so, and each partition dividing it into compartments is taller than its predecessor, so that the whole of the solid in (1) must be decomposed, and the compartment entirely filled with liquid before it can overflow into (2), and so on. Since the carbide in all the later receptacles is exposed to the water vapour produced in that one in which decomposition is proceeding at any given moment, at least at its upper surface, some after-generation between vapour and carbide occurs in H; but a partial control over the temperature may be obtained by water-jacketing the container. In G the water enters at the base and gas escapes at the top, the carbide vessels being disposed vertically; hero, perhaps, more after- generation of the same description occurs, as the moist gas streams round and over the higher baskets. In K, the water enters at the top and must completely fill basket (1) before it can run down the depending pipe into (2); but since the gas also leaves the generator at the top, the later carbide receptacles do not come in contact with water vapour, but are left practically unattacked until their time arrives for decomposition by means of liquid water. K, therefore, is the best arrangement of parts to avoid after-generation, overheating, and polymerisation of the acetylene whether the generator be worked as a contact or as a flooded-compartment apparatus; but it may be freely admitted that the extent of the overheating due to reaction between water vapour and carbide may be kept almost negligible in either K, H, or G, provided the partitions in the carbide container be sufficient in number—provided, that is to say, that each compartment holds a sufficiently small quantity of carbide; and provided that the quantity of water ultimately required to fill each compartment is relatively so large that the temperature of the liquid never approaches the boiling-point where vaporisation is rapid. The type of generator indicated by K has not become very popular, but G is fairly common, whilst H undoubtedly represents the apparatus which is most generally adopted for use in domestic and other private installations in the United Kingdom and the Continent of Europe. The actual generators made according to the design shown by H usually have a carbide receptacle designed in the form of a semi-cylindrical or rectangular vessel of steel sliding fairly closely into an outside container, the latter being either built within the main water space of the entire apparatus or placed within a separate water-jacketed casing. Owing to its shape and the sliding motion with which the carbide receptacle is put into the container these generators are usually termed "drawer" generators. In comparison with type G, the drawer generator H certainly exhibits a lower rise in temperature when gas is evolved in it at a given speed and when the carbide receptacles are constructed of similar dimensions. It is very desirable that the whole receptacle should be subdivided into a sufficient number of compartments and that it should be effectively water-cooled from outside. It would also be advantageous if the water- supply were so arranged that the generator should be a true flooded- compartment apparatus, but experience has nevertheless shown that generators of type H do work very well when the water admitted to the carbide receptacle, each time the feed comes into action, is not enough to flood the carbide in one of the compartments. Above a certain size drawer generators are usually constructed with two or even more complete decomposing vessels, arrangements being such that one drawer can be taken out for cleaning, whilst the other is in operation. When this is the case a third carbide receptacle should always be employed so that it may be dry, lit to receive a charge of carbide, and ready to insert in the apparatus when one of the others is withdrawn. The water-feed should always be so disposed that the attendant can see at a glance which of the two (or more) carbide receptacles is in action at any moment, and it should be also so designed that the supply is automatically diverted to the second receptacle when the first is wholly exhausted and back again to the first (unless there are more than two) when the carbide in the second is entirely gasified. In the sketches G, H, and K, the total space occupied by the various carbide receptacles is represented as being considerably smaller than the capacity of the decomposing chamber. Were this method of construction copied in actual acetylene apparatus, the first makes of gas would be seriously (perhaps dangerously) contaminated with air. In practice the receptacles should fit so tightly into the outer vessel and into one another that when loaded to the utmost extent permissible—space being left for the swelling of the charge and for the passage of water and gas—but little room should be left for the retention of air in the chamber.

ACTION OF CARBIDE-TO-WATER GENERATORS.—The methods which may be adopted to render a generator automatic when carbide is employed as the moving material are shown at M, N, and P, in Fig. 6; but the precise devices used in many actual apparatus are so various that it is difficult to portray them generically. Moreover it is desirable to subdivide automatic carbide-to-water generators, according to the size of the carbide they are constructed to take, into two or three classes, which are termed respectively "large carbide-feed," "small carbide-feed," and "granulated carbide-feed" apparatus. (The generator represented at L does not really belong to the present class, being non-automatic and fed by hand; but the sketch is given for completeness.) M is an automatic carbide-feed generator having its store of carbide in a hopper carried by the rising- holder bell. The hopper is narrowed at its mouth, where it is closed by a conical or mushroom valve d supported on a rod held in suitable guides. When the bell falls by consumption of gas, it carries the valve and rod with it; but eventually the button at the base of c strikes the bottom of the generator, or some fixed distributing plate, and the rod can descend no further. Then, when the bell falls lower, the mushroom d rises from its seat, and carbide drops from the hopper into the water. This type of apparatus has the defect characteristic of A^2, Fig. 1; for the pressure in the service steadily diminishes as the effective weight of bell plus hopper decreases by consumption of carbide. But it has also two other defects—(1) that ordinary carbide is too irregular in shape to fall smoothly through the narrow annular space between the valve and its seat; (2) that water vapour penetrates into the hopper, and liberates some gas there, while it attacks the lumps of carbide at the orifice, producing dust or causing them to stick together, and thus rendering the action of the feed worse than ever. Most of these defects can be avoided by using granulated carbide, which is more uniform in size and shape, or by employing a granulated and "treated" carbide which has been dipped in some non-aqueous liquid to make it less susceptible to the action of moisture. Both these plans, however, are expensive to adopt; first, because of the actual cost of granulating or "treating" the carbide; secondly, because the carbide deteriorates in gas-making capacity by its inevitable exposure to air during the granulating or "treating" process. The defects of irregularity of pressure and possible waste of gas by evolution in the hopper may be overcome by disposing the parts somewhat differently; making the holder an annulus round the hopper, or making it cylindrical with the hopper inside. In this case the hopper is supported by the main portion of the apparatus, and does not move with the bell: the rod and valve being given their motion in some fashion similar to that figured. Apparatus designed in accordance with the sketch M, or with the modification just described, are usually referred to under the name of "hopper" generators. On several occasions trouble has arisen during their employment owing to the jamming of the valve, a fragment of carbide rather larger than the rest of the material lodging between the lips of the hopper and the edges of the mushroom valve. This has been followed by a sudden descent of all the carbide in the store into the water beneath, and the evolution of gas has sometimes been too rapid to pass away at the necessary speed into the holder. The trouble is rendered even more serious should the whole charge of carbide fall at a time when, by neglect or otherwise, the body of the generator contains much lime sludge, the decomposition then proceeding under exceptionally bad circumstances, which lead to the production of an excessively high temperature. Hopper generators are undoubtedly very convenient for certain purposes, chiefly, perhaps, for the construction of table-lamps and other small installations. Experience tends to show that they may be employed, first, provided they are designed to take granulated carbide—which in comparison with larger grades is much more uniform and cylindrical in shape—and secondly, provided the quantity of carbide in the hopper does not exceed a few pounds. The phenomenon of the sudden unexpected descent of the carbide, popularly known as "dumping," can hardly be avoided with carbide larger in size than the granulated variety; and since the results of such an accident must increase in severity with the size of the apparatus, a limit in their capacity is desirable.

When it is required to construct a carbide-feed generator of large size or one belonging to the large carbide-feed pattern, it is preferable to arrange the store in a different manner. In N the carbide is held in a considerable number of small receptacles, two only of which are shown in the drawing, provided with detachable lids and hinged bottoms kept shut by suitable catches. At proper intervals of time those catches in succession are knocked on one side by a pin, and the contents of the vessel fall into the water. There are several methods available for operating the pins. The rising-holder bell may be made to actuate a train of wheels which terminate in a disc revolving horizontally on a vertical axis somewhere just below the catches; and this wheel may bear an eccentric pin which hits each catch as it rotates. Alternatively the carbide boxes may be made to revolve horizontally on a vertical axis by the movements of the bell communicated through a clutch; and thus each box in succession may arrive at a certain position where the catch is knocked aside by a fixed pin. The boxes, again, may revolve vertically on a horizontal axis somewhat like a water-wheel, each box having its bottom opened, or, by a different system of construction, being bodily upset, when it arrives at the bottom of its circular path. In no case, however, are the carbide receptacles carried by the bell, which is a totally distinct part of the apparatus; and therefore in comparison with M, the pressure given by the bell is much more uniform. Nevertheless, if the system of carbide boxes moves at all, it becomes easier to move by decrease in weight and consequent diminution in friction as the total charge is exhausted; and accordingly the bell has less work to do during the later stages of its operation. For this reason the plan actually shown at N is preferable, since the work done by the moving pin, i.e., by the descending bell, is always the same. P represents a carbide-feed effected by a spiral screw or conveyor, which, revolved periodically by a moving bell, draws carbide out of a hopper of any desired size and finally drops it into a shoot communicating with a generating chamber such as that shown in L. Here the work done by the bell is large, as the friction against the blades of the screw and the walls of the horizontal tube is heavy; but that amount of work must always be essentially identical. The carbide-feed may similarly be effected by means of some other type of conveyor instead of the spiral screw, such as an endless band, and the friction in these cases may be somewhat less than with the screw, but the work to be done by the bell will always remain large, whatever type of conveyor may be adopted. A further plan for securing a carbide-feed consists in employing some extraneous driving power to propel a charge of carbide out of a reservoir into the generator. Sometimes the propulsive effort is obtained from a train of clockwork, sometimes from a separate supply of water under high pressure. The clockwork or the water power is used either to drive a piston travelling through the vessel containing the carbide so that the proper quantity of material is dropped over the open mouth of a shoot, or to upset one after another a series of carbide receptacles, or to perform some analogous operation. In these cases the pin or other device fitted to the acetylene apparatus itself has nothing to do beyond releasing the mechanism in question, and therefore the work required from the bell is but small. The propriety of employing a generator belonging to these latter types must depend upon local conditions, e.g., whether the owner of the installation has hydraulic power on a small scale (a constant supply of water under sufficient pressure) at disposal, or whether he does not object to the extra labour involved in the periodical winding up of a train of clockwork.

It must be clear that all these carbide-feed arrangements have the defect in a more or less serious degree of leaving the carbide in the main storage vessel exposed to the attack of water vapour rising from the decomposing chamber, for none of the valves or operating mechanism can be made quite air-tight. Evolution of gas produced in this way does not matter in the least, because it is easy to return the gas so liberated into the generator or into the holder; while the extent of the action, and the consequent production of overheating, will tend to be less than in generators such as those shown in G and H of Figs. 4 and 5, inasmuch as the large excess of water in the carbide-feed apparatus prevents the liquid arriving at a temperature at which it volatilises rapidly. The main objection to the evolution of gas in the carbide vessel of a carbide-to-water generator depends on the danger that the smooth working of the feed-gear may be interfered with by the formation of dust or by the aggregation of the carbide lumps.

USE OF OIL IN GENERATORS.—Calcium carbide is a material which is only capable of attack for the purpose of evolving acetylene by a liquid that is essentially water, or by one that contains some water mixed with it. Oils and the like, or even such non-aqueous liquids as absolute alcohol, have no effect upon carbide, except that the former naturally make it greasy and somewhat more difficult to moisten. This last property has been found of service in acetylene generation, especially on the small scale; for if carbide is soaked in, or given a coating of, some oil, fat, or solid hydrocarbon like petroleum, cocoanut oil, or paraffin wax, the substance becomes comparatively indifferent towards water vapour or the moisture present in the air, while it still remains capable of complete, albeit slow, decomposition by liquid water when completely immersed therein. The fact that ordinary calcium carbide is attacked so quickly by water is really a defect of the substance; for it is to this extreme rapidity of reaction that the troubles of overheating are due. Now, if the basket in the generator B^1 of Fig. 2, or, indeed, the carbide store in any of the carbide-to-water apparatus, is filled with a carbide which has been treated with oil or wax, as long as the water-level stands at l' and l" or the carbide still remains in the hopper, it is essentially unattacked by the vapour arising from the liquid; but directly the basket is submerged, or the lumps fall into the water, acetylene is produced, and produced more slowly and regularly than otherwise. Again, oils do not mix with water, but usually float thereon, and a mass of water covered by a thick film or layer of oil does not evaporate appreciably. If, now, a certain quantity of oil, say lamp paraffin or mineral lubricating oil, is poured on to the water in B^1, Fig. 2, it moves upwards and downwards with the water. When the water takes the position l, the oil is driven upwards away from the basket of carbide, and acetylene is generated in the ordinary manner; but when the water falls to l" the oil descends also, rinses off much of the adhering water from the carbide lumps, covers them with a greasy film, and almost entirely stops generation till it is in turn washed off by the next ascent of the water. Similarly, if the carbide in generators F, G, and H (also K) has been treated with a solid or semi-solid grease, it is practically unattacked by the stream of warm damp gas, and is only decomposed when the liquid itself arrives in the basket. For the same reason treated carbide can be kept for fairly long periods of time, even in a drum with badly fitting lid, without suffering much deterioration by the action of atmospheric moisture. The problem of acetylene generation is accordingly simplified to a considerable degree by the use of such treated carbide, and the advantage becomes more marked as the plant decreases in size till a portable apparatus is reached, because the smaller the installation the more relatively expensive or inconvenient is a large holder for surplus gas. The one defect of the method is the extra cost of such treated carbide; and in English conditions ordinary calcium carbide is too expensive to permit of any additional outlay upon the acetylene if it is to compete with petroleum or the product of a tiny coal-gas works. The extra cost of using treated carbide falls upon the revenue account, and is much more noticeable than that of a large holder, which is capital expenditure. When fluid oil is employed in a generator of type B^1, evolution of gas becomes so regular that any holder beyond the displacement one which the apparatus itself constitutes is actually unnecessary, though still desirable; but B^1, with or without oil, still remains a displacement apparatus, and as such gives no constant pressure. It must be admitted that the presence of oil so far governs the evolution of gas that the movement of the water, and the consequent variation of pressure, is rendered very small; still a governor or a rising holder would be required to give the best result at the burners. One point in connexion with the use of liquid oil must not be overlooked, viz., the extra trouble it may give in the disposal of the residues. This matter will be dealt with more fully in Chapter V.; here it is sufficient to say that as the oil does not mix with the water but floats on the surface, care has to be taken that it is not permitted to enter any open stream. The foregoing remarks about the use of oil manifestly only apply to those cases where it is used in quantity and where it ultimately becomes mixed with the sludge or floats on the water in the decomposing chamber. The employment of a limpid oil, such as paraffin, as an intermediate liquid into which carbide is introduced on its way to the water in the decomposing vessel of a hand-fed generator in the manner described on page 70 is something quite different, because, except for trifling losses, one charge of oil should last indefinitely.

RISING GASHOLDERS.—Whichever description of holder is employed in an acetylene apparatus, the gas is always stored over, or in contact with, a liquid that is essentially water. This introduces three subjects for consideration: the heavy weight of a large body of liquid, the loss of gas by dissolution in that liquid, and the protection of that liquid from frost in the winter. The tanks of rising holders are constructed in two different ways. In one the tank is a plain cylindrical vessel somewhat larger in diameter than the bell which floats in it; and since there must be nearly enough water in the tank to fill the interior of the bell when the latter assumes its lowest position, the quantity of water is considerable, its capacity for dissolving acetylene is large, and the amount of any substance that may have to be added to it to lower its freezing-point becomes so great as to be scarcely economical. All these defects, including that of the necessity for very substantial foundations under the holder to support its enormous weight, may be overcome by adopting the second method of construction. It is clear that the water in the centre of the tank is of no use,—all that is needed being a narrow trough for the bell to work in. Large rising holders are therefore advantageously built with a tank formed in the shape of an annulus, the effective breadth of which is not more than 2 or 3 inches, the centre portion being roofed over so as to prevent escape of gas. The same principle may be retained with modified details by fitting inside a plain cylindrical tank a "dummy" or smaller cylinder, closed by a flat or curved top and fastened water- and air-tight to the bottom of the main vessel. The construction of annular tanks or the insertion of a "dummy" may be attended with difficulty if the tank is wholly or partly sunk below the ground level, owing to the lifting force of water in the surrounding soil. Where a steel tank is sunk, or a masonry tank is constructed, regard must be paid, both in the design of the tank and in the manner of construction, to the level of the underground water in the neighbourhood, as in certain cases special precautions will be needed to avoid trouble from the pressure of the water on the outside of the tank until it is balanced by the pressure of the water with which the tank is filled. So far as mere dissolution of gas is concerned, the loss may be reduced by having a circular disc of wood, &c., a little smaller in diameter than the boll, floating on the water of a plain tank.

EFFECT OF STORAGE IN GASHOLDER ON ACETYLENE.—It is perfectly true, as has been stated elsewhere, that the gas coming from an acetylene generator loses some of its illuminating power if it is stored over water for any great length of time; such loss being given by Nichols as 94 per cent, in five months, and having been found by one of the authors as 0.63 per cent. per day—figures which stand in fair agreement with one another. This wastage is not due to any decomposition of the acetylene in contact with water, but depends on the various solubilities of the different gases which compose the product obtained from commercial calcium carbide. Inasmuch as an acetylene evolved in the best generator contains some foreign ingredients, and inasmuch as an inferior product contains more (cf. Chapter V.), the contents of a holder are never pure; but as those contents are principally made up of acetylene itself, that gas stands at a higher partial pressure in the holder than the impurities. Since acetylene is more soluble in water than any of its diluents or impurities, sulphuretted hydrogen and ammonia excepted, and since the solubility of all gases increases as the pressure at which they are stored rises, the true acetylene in an acetylene holder dissolves in the water more rapidly and comparatively more copiously than the impurities; and thus the acetylene tends to disappear and the impurities to become concentrated within the bell. Simultaneously at the outer part of the seal, air is dissolved in the water; and by processes of diffusion the air so dissolved passes through the liquid from the outside to the inside, where it escapes into the bell, while the dissolved acetylene similarly passes from the inside to the outside of the seal, and there mingles with the atmosphere. Thus, the longer a certain volume of acetylene is stored over water, the more does it become contaminated with the constituents of the atmosphere and with the impurities originally present in it; while as the acetylene is much more soluble than its impurities, more gas escapes from, than enters, the holder by diffusion, and so the bulk of stored gas gradually diminishes. However, the figures previously given show that this action is too slow to be noticeable in practice, for the gas is never stored for more than a few days at a time. The action cannot be accepted as a valid argument against the employment of a holder in acetylene plant. Such deterioration and wastage of gas may be reduced to some extent by the use of a film of some cheap and indifferent oil floating on the water inside an acetylene holder; the economy being caused by the lower solubility of acetylene in oils than in aqueous liquids not saturated with some saline material. Probably almost any oil would answer equally well, provided it was not volatile at the temperature of the holder, and that it did not dry or gum on standing, e.g., olive oil or its substitutes; but mineral lubricating oil is not so satisfactory. It is, however, not necessary to adopt this method in practice, because the solvent power of the liquid in the seal can be reduced by adding to it a saline body which simultaneously lowers its freezing-point and makes the apparatus more trustworthy in winter.

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