Nitro-Explosives: A Practical Treatise
by P. Gerald Sanford
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Public Analyst to the Borough of Penzance; late Consulting Chemist to the Cotton Powder Company Limited; and formerly Resident Chemist at the Stowmarket Works of the New Explosives Company Limited, and the Hayle Works of the National Explosive Company Limited

Second Edition, Revised and Enlarged


In compiling the following treatise, my aim has been to give a brief but thoroughly practical account of the properties, manufacture, and methods of analysis of the various nitro-explosives now so largely used for mining and blasting purposes and as propulsive agents; and it is believed that the account given of the manufacture of nitro-glycerine and of the gelatine dynamites will be found more complete than in any similar work yet published in this country.

For many of the facts and figures contained in the chapter on Smokeless Powders I am indebted to (amongst others) the late Mr J.D. Dougall and Messrs A.C. Ponsonby and H.M. Chapman, F.C.S.; and for details with regard to Roburite to Messrs H.A. Krohn and W.J. Orsman, F.I.C. To these gentlemen my cordial thanks are due. Among the authorities which have been consulted in the general preparation of the work may be mentioned the Journals of the Chemical Society, the Society of Chemical Industry, the United States Naval Institute, and the Royal Artillery Institution. I have also referred to several volumes of the periodical publication Arms and Explosives; to various papers by Sir Frederick Abel, Bart., F.R.S., and General Wardell, R.A., on Gun-Cotton; to "Modern Artillery," by Capt. Lloyd, R.N., and A.G. Hadcock, R.A.; to the late Colonel Cundill's "Dictionary of Explosives"; as well as to the works of Messrs Eissler, Berthelot, and others.

The illustrations have been prepared chiefly from my own drawings. A few, however, have been taken (by permission) from the pages of Arms and Explosives, or from other sources which are acknowledged in the text.




May 1896.


In the preparation of the Second Edition of this work, I have chiefly made use of the current technical journals, especially of the Journal of the Society of Chemical Industry. The source of my information has in every case been acknowledged.

I am also indebted to several manufacturers of explosives for information respecting their special products—among others the New Explosives Company Ltd.; Messrs Curtis's and Harvey Ltd.; The Schultze Gunpowder Company Ltd.; and Mr W.D. Borland, F.I.C., of the E.C. Powder Company Ltd.

To my friend Mr A. Stanley Fox, F.C.S., of Faversham, my best thanks are also due for his help in many departments, and his kindness in pointing out several references.

The chapter on Smokeless Powders has been considerably enlarged and (as far as possible) brought up to date; but it has not always been possible to give the process of manufacture or even the composition, as these details have not, in several cases, been made public.


LONDON, June 1906.



The Nitro-Explosives—Substances that have been Nitrated—The Danger Area— Systems of Professors Lodge, Zenger, and Melsens for the Protection of Buildings from Lightning, &c.


Properties of Nitro-Glycerine—Manufacture—Nitration—Separation—Washing and Filtering—Drying, Storing, &c.—The Waste Acids—Their Treatment— Nitric Acid Plants


Cellulose Properties—Discovery of Gun-Cotton—Properties of Gun-Cotton— Varieties of Soluble and Insoluble Gun-Cottons—Manufacture of Gun-Cotton— Dipping and Steeping—Whirling Out the Acid—Washing, Boiling, Pulping, Compressing—The Waltham Abbey Process—Le Bouchet Process—Granulation of Gun-Cotton—Collodion-Cotton—Manufacture—Acid Mixture Used—Cotton Used, &c.—Nitrated Gun-Cotton—Tonite—Dangers in Manufacture of Gun-Cotton— Trench's Fire-Extinguishing Compound—Uses of Collodion-Cotton—Celluloid— Manufacture, &c.—Nitro-Starch, Nitro-Jute, and Nitro-Mannite


Kieselguhr Dynamite—Classification of Dynamites—Properties and Efficiency of Ordinary Dynamite—Other forms of Dynamite—Gelatine and Gelatine Dynamites, Suitable Gun-Cotton for, and Treatment of—Other Materials Used—Composition of Gelignite—Blasting Gelatine—Gelatine Dynamite—Absorbing Materials—Wood Pulp—Potassium Nitrate, &c.— Manufacture, &c.—Apparatus Used—The Properties of the Gelatine Compounds


Explosives derived from Benzene—Toluene and Nitro-Benzene—Di- and Tri-nitro-Benzene—Roburite: Properties and Manufacture—Bellite: Properties, &c.—Securite—Tonite No. 3.—Nitro-Toluene— Nitro-Naphthalene—Ammonite—Sprengel's Explosives—Picric Acid— Picrates—Picric Powders—Melinite—Abel's Mixture—Brugere's Powders— The Fulminates—Composition, Formula, Preparation, Danger of, &c.— Detonators: Sizes, Composition, Manufacture—Fuses, &c.


Composition, Formula, Preparation, Danger of, &c.—Detonators: Sizes, Composition, Manufacture—Fuses, &c.


Cordite—Axite—Ballistite—U.S. Naval Powder—Schultze's E.C. Powder— Indurite—Vielle Poudre—Walsrode and Cooppal Powders—Amberite— Troisdorf—B.N. Powder—Wetterin—Normal Powder—Maximite—Picric Acid Powders, &c. &c.


Kieselguhr Dynamite—Gelatine Compounds—Tonite—Cordite—Vaseline— Acetone—Scheme for Analysis of Explosives—Nitro-Cotton—Solubility Test— Non-Nitrated Cotton—Alkalinity—Ash and Inorganic Matter—Determination of Nitrogen—Lunge, Champion and Pellet's, Schultze-Tieman, and Kjeldahl's Methods—Celluloid—Picric Acid and Picrates—Resinous and Tarry Matters— Sulphuric Acid and Hydrochloric Acid and Oxalic Acid—Nitric Acid— Inorganic Impurities—General Impurities and Adulterations—Potassium Picrate, &c.—Picrates of the Alkaloids—Analysis of Glycerine—Residue— Silver Test—Nitration—Total Acid Equivalent—Neutrality—Free Fatty Acids—Combined Fatty Acids—Impurities—Oleic Acid—Sodium Chloride— Determination of Glycerine—Waste Acids—Sodium Nitrate—Mercury Fulminate—Cap Composition—Table for Correction of Volumes of Gases, for Temperature and Pressure


Horsley's Apparatus—Table of Firing Points—The Government Heat Test Apparatus, &c., for Dynamites, Nitro-Glycerine, Nitro-Cotton, and Smokeless Powders—Guttmann's Heat Test—Liquefaction and Exudation Tests— Page's Regulator for Heat Test Apparatus—Specific Gravities of Explosives—Will's Test for Nitro-Cellulose—Table of Temperature of Detonation, Sensitiveness, &c.


Effectiveness of an Explosive—High and Low Explosives—Theoretical Efficiency—M.M. Roux and Sarrau's Results—Abel and Noble's—Nobel's Ballistic Test—The Mortar—Pressure or Crusher Gauge—Calculation Volume of Gas Evolved, &c.—Lead Cylinders—The Foot-Pounds Machine—Noble's Pressure Gauge—Lieut. Walke's Results—Calculation of Pressure Developed by Dynamite and Gun-Cotton—McNab's and Ristori's Results of Heat Developed by the Explosion of Various Explosives—Composition of some of the Explosives in Common Use for Blasting, &c.



FRONTISPIECE—Danger Building showing Protecting Mounds. 1. Section of Nitro-Glycerine Conduit 2. Melsens System of Lightning Conductors 3. French System 4a & 4b. English Government System 5. Upper Portion of Nitrator for Nitro-Glycerine 6. Small Nitrator 7. Nathan's Nitrator 8. Nitro-Glycerine Separator 9. Nitro-Glycerine Filtering Apparatus 10. Cotton-Waste Drier 11. Dipping Tank 12. Cooling Pits 13. Steeping Pot for Gun-Cotton 14. Hydro-Extractor or Centrifugal Drier 15a & 15b. Gun-Cotton Beater 16a. Poacher for Pulping Gun-Cotton 16b. Plan of same 16c. Another form of Poacher 17 & 18. Compressed Gun-Cotton 19. Hydraulic Press 20. Thomson's Apparatus—Elevation 21. Elevation Plan 22. Trench's Safety Cartridge 23. Vessel used in Nitrating Paper 24. Cage ditto—White & Schupphaus' Apparatus 25. Do. do. do. 26 & 27. Nitrating Pot for Celluloid 28 & 29. Plunge Tank in Plan and Section 30. Messrs Werner, Pfleiderer & Perkins' Mixing Machine 31. M. 'Roberts' Mixing Machine for Blasting Gelatine 32. Plan of same 33. Cartridge Machine for Gelatines 34. Cartridge fitted with Fuse and Detonator 35. Gun-Cotton Primer 36. Electric Firing Apparatus 37. Metal Drum for Winding Cordite 38. Ten-Stranding 39. Curve showing relation between Pressures of Cordite and Black Powder, by Professor Vivian Lewes 40. Marshall's Apparatus for Moisture in Cordite 41. Lunge's Nitrometer 42. Modified do. 43. Horn's Nitrometer 44. Schultze-Tieman Apparatus for Determination of Nitrogen in Gun-Cotton 45. Decomposition Flask for Schultze-Tieman Method 46. Abel's Heat Test Apparatus 47. Apparatus for Separation of Nitro-Glycerine from Dynamite 48. Test Tube arranged for Heat Test 49. Page's Regulator 50. Do. showing Bye-Pass and Cut-off Arrangement 51. Will's Apparatus 52 & 53. Curves obtained 54. Dynamite Mortar 55. Quinan's Pressure Gauge 56. Steel Punch and Lead Cylinder for Use with Pressure Gauge 57. Micrometer Calipers for Measuring Thickness of Lead Cylinders 58. Section of Lead Cylinders before and after Explosion 59. Noble's Pressure Gauge 60. Crusher Gauge




The Nitro-Explosives—Substances that have been Nitrated—The Danger Area— Systems of Professors Lodge, Zenger, and Melsens for the Protection of Buildings from Lightning, &c.

The manufacture of the various nitro-explosives has made great advances during late years, and the various forms of nitro-compounds are gradually replacing the older forms of explosives, both for blasting purposes and also for propulsive agents, under the form of smokeless powders. The nitro-explosives belong to the so-called High Explosives, and may be defined as any chemical compound possessed of explosive properties, or capable of combining with metals to form an explosive compound, which is produced by the chemical action of nitric acid, either alone or mixed with sulphuric acid, upon any carbonaceous substance, whether such compound is mechanically mixed with other substances or not.[A]

[Footnote A: Definition given in Order of Council, No. 1, Explosives Act, 1875.]

The number of compounds and mixtures included under this definition is very large, and they are of very different chemical composition. Among the substances that have been nitrated are:—Cellulose, under various forms, e.g., cotton, lignin, &c.; glycerine, benzene, starch, jute, sugar, phenol, wood, straw, and even such substances as treacle and horse-dung. Some of these are not made upon the large scale, others are but little used. Those of most importance are nitro-glycerine and nitro-cellulose. The former enters into the composition of all dynamites, and several smokeless powders; and the second includes gun-cotton, collodion-cotton, nitrated wood, and the majority of the smokeless powders, which consist generally of nitro-cotton, nitro-lignin, nitro-jute, &c. &c., together with metallic nitrates, or nitro-glycerine.

The nitro-explosives consist generally of some organic substance in which the NO_{2} group, known as nitryl, has been substituted in place of hydrogen.

Thus in glycerine,

OH C{3}H{5} OH, OH

which is a tri-hydric alcohol, and which occurs very widely distributed as the alcoholic or basic constituent of fats, the hydrogen atoms are replaced by the NO_{2} group, to form the highly explosive compound, nitro-glycerine. If one atom only is thus displaced, the mono-nitrate is formed thus,

ONO_{2} C_{3}H_{5} OH; OH

and if the three atoms are displaced, C{3}H{5}(ONO{2}){3}, or the tri- nitrate, is formed, which is commercial nitro-glycerine.

Another class, the nitro-celluloses, are formed from cellulose, C{6}H{10}O{5}, which forms the groundwork of all vegetable tissues. Cellulose has some of the properties of the alcohols, and forms ethereal salts when treated with nitric and sulphuric acids. The hexa-nitrate, or gun-cotton, has the formula, C{12}H{14}O{4}(ONO{2}){6}; and collodion-cotton, pyroxylin, &c., form the lower nitrates, i.e., the tetra- and penta-nitrates. These last are soluble in various solvents, such as ether-alcohol and nitro-glycerine, in which the hexa-nitrate is insoluble. They all dissolve, however, in acetone and acetic ether.

The solution of the soluble varieties in ether-alcohol is known as collodion, which finds many applications in the arts. The hydrocarbon benzene, C{6}H{6}, prepared from the light oil obtained from coal-tar, when nitrated forms nitro-benzenes, such as mono-nitro-benzene, C{6}H{5}NO{2}, and di-nitro-benzene, C{6}H{4}(NO{2}){2}, in which one and two atoms are replaced by the NO{2} group. The latter of these compounds is used as an explosive, and enters into the composition of such well-known explosives as roburite, &c. The presence of nitro groups in a substance increases the difficulty of further nitration, and in any case not more than three nitro groups can be introduced into an aromatic compound, or the phenols. All aromatic compounds with the general formula, C{6}H{4}X{2}, give, however, three series. They are called ortho, meta, or para compounds, depending upon the position of NO{2} groups introduced.

Certain regularities have been observed in the formation of nitro- compounds. If, for example, a substance contains alkyl or hydroxyl groups, large quantities of the para compound are obtained, and very little of the ortho. The substitution takes place, however, almost entirely in the meta position, if a nitro, carboxyl, or aldehyde group be present. Ordinary phenol, C{6}H{5}.OH, gives para- and ortho-nitro-phenol; toluene gives para- and ortho-nitro-toluene; but nitro-benzene forms meta-di-nitro- benzene and benzoic acid, meta-nitro-benzoic acid.[A]

[Footnote A: "Organic Chemistry," Prof. Hjelt. Translated by J.B. Tingle, Ph.D.]

If the graphic formula of benzene be represented thus (No. 1), then the positions 1 and 2 represent the ortho, 1 and 3 the meta, and 1 and 4 the para compounds. When the body phenol, C{6}H{5}.OH, is nitrated, a compound is formed known as tri-nitro-phenol, or picric acid, C{6}H{2}(NO{2}){3}OH, which is used very extensively as an explosive, both as picric acid and in the form of picrates. Another nitro body that is used as an explosive is nitro-naphthalene, C{10}H{6}(NO{2}){2}, in roburite, securite, and other explosives of this class. The hexa-nitro- mannite, C{6}H{8}(ONO{2}){6}, is formed

by treating a substance known as mannite, C_{6}H_{8}(OH)_{6}, an alcohol formed by the lactic acid fermentation of sugar and closely related to the sugars, with nitric and sulphuric acids. It is a solid substance, and very explosive; it contains 18.58 per cent. of nitrogen.

Nitro-starch has also been used for the manufacture of an explosive. Muhlhauer has described (_Ding. Poly. Jour._, 73, 137-143) three nitric ethers of starch, the tetra-nitro-starch, C_{12}H_{16}O_{6}(ONO_{2})_{4}, the penta- and hexa-nitro-starch. They are formed by acting upon potato starch dried at 100 deg. C. with a mixture of nitric and sulphuric acids at a temperature of 20 deg. to 25 deg. C. Rice starch has also been used in its production. Muhlhauer proposes to use this body as a smokeless powder, and to nitrate it with the spent mixed acids from the manufacture of nitro- glycerine. This substance contains from 10.96 to 11.09 per cent. of nitrogen. It is a white substance, very stable and soluble even in cold nitro-glycerine.

The explosive bodies formed by the nitration of jute have been studied by Messrs Cross and Bevan. and also by Muehlhaeuer. The former chemists give jute the formula C_{12}H_{18}O_{9}, and believe that its conversion into a nitro-compound takes place according to the equation—

C{12}H{18}O{9} + 3HNO{3} = 3H{2}O + C{12}H{15}O(6}(NO{3}){3}.

This is equivalent to a gain in weight of 44 per cent. for the tri- nitrate, and 58 per cent. for the tetra-nitrate. The formation of the tetra-nitrate appears to be the limit of nitration of jute fibre. Messrs Cross and Bevan say, "In other words, if we represent the ligno-cellulose molecule by a C_{12} formula, it will contain four hydroxyl (OH) groups, or two less than cellulose similarly represented." It contains 11.5 per cent. of nitrogen. The jute nitrates resemble those of cellulose, and are in all essential points nitrates of ligno-cellulose.

Nitro-jute is used in the composition of the well-known Cooppal Smokeless Powders. Cross and Bevan are of opinion that there is no very obvious advantage in the use of lignified textile fibres as raw materials for explosive nitrates, seeing that a number of raw materials containing cellulose (chiefly as cotton) can be obtained at from L10 to L25 a ton, and yield also 150 to 170 per cent. of explosive material when nitrated (whereas jute only gives 154.4 per cent.), and are in many ways superior to the products obtained from jute. Nitro-lignin, or nitrated wood, is, however, largely used in the composition of a good many of the smokeless powders, such as Schultze's, the Smokeless Powder Co.'s products, and others.

The Danger Area.—That portion of the works that is devoted to the actual manufacture or mixing of explosive material is generally designated by the term "danger area," and the buildings erected upon it are spoken of as "danger buildings." The best material of which to construct these buildings is of wood, as in the event of an explosion they will offer less resistance, and will cause much less danger than brick or stone buildings. When an explosion of nitro-glycerine or dynamite occurs in one of these buildings, the sides are generally blown out, and the roof is raised some considerable height, and finally descends upon the blown-out sides. If, on the other hand, the same explosion had occurred in a strong brick or stone building, the walls of which would offer a much larger resistance, large pieces of brickwork would probably have been thrown for a considerable distance, and have caused serious damage to surrounding buildings.

It is also a very good plan to surround all danger buildings with mounds of sand or earth, which should be covered with turf, and of such a height as to be above the roof of the buildings that they are intended to protect (see frontispiece).[A] These mounds are of great value in confining the force of the explosion, and the sides of the buildings being thrown against them are prevented from travelling any distance. In gunpowder works it is not unusual to surround the danger buildings with trees or dense underwood instead of mounds. This would be of no use in checking the force of explosion of the high explosives, but has been found a very useful precaution in the case of gunpowder.

[Footnote A: At the Baelen Factory, Belgium, the danger buildings are erected on a novel plan. They are circular in ground plan and lighted entirely from the roof by means of a patent glass having wire-netting in it, and which it is claimed will not let a splinter fall, even if badly cracked. The mounds are then erected right up against the walls of the building, exceeding them in height by several metres. For this method of construction it is claimed that the force exerted by an explosion will expand itself in a vertical direction ("Report on Visits to Certain Explosive Factories," H.M. Inspectors, 1905).]

In Great Britain it is necessary that all danger buildings should be a specified distance apart; a license also must be obtained. The application for a license must give a plan (drawn to scale) of the proposed factory or magazine, and the site, its boundaries, and surroundings, and distance the building will be from any other buildings or works, &c., also the character, and construction of all the mounds, and nature of the processes to be carried on in the factory or building.[A]

[Footnote A: Explosives Act, 38 Vict. ch. 17.]

The selection of a site for the danger area requires some attention. The purpose for which it is required, that is, the kind of explosive that it is intended to manufacture, must be taken into consideration. A perfectly level piece of ground might probably be quite suitable for the purpose of erecting a factory for the manufacture of gun-cotton or gunpowder, and such materials, but would be more or less unsuitable for the manufacture of nitro-glycerine, where a number of buildings are required to be upon different levels, in order to allow of the flow of the liquid nitro- glycerine from one building to another through a system of conduits. These conduits (Fig. 1), which are generally made of wood and lined with lead, the space between the woodwork and the lead lining, which is generally some 4 or 5 inches, being filled with cinders, connect the various buildings, and should slope gently from one to the other. It is also desirable that, as far as possible, they should be protected by earth-work banks, in the same way as the danger buildings themselves. They should also be provided with covers, which should be whitewashed in hot weather.

A great deal of attention should be given to these conduits, and they should be very frequently inspected. Whenever it is found that a portion of the lead lining requires repairing, before cutting away the lead it should be very carefully washed, for several feet on either side of the portion that it is intended to remove, with a solution of caustic soda or potash dissolved in methylated spirit and water, and afterwards with water alone. This decomposes the nitro-glycerine forming glycerine and potassium nitrate. It will be found that the mixed acids attack the lead rather quickly, forming sulphate and nitrate of lead, but chiefly the former. It is on this account that it has been proposed to use pipes made of guttapercha, but the great drawback to their use is that in the case of anything occurring inside the pipes, such as the freezing of the nitro- glycerine in winter, it is more difficult to find it out, and the condition of the inside cannot be seen, whereas in the case of wooden conduits it is an easy matter to lift the lids along the whole length of the conduit.

The buildings which require to be connected by conduits are of course those concerned with the manufacture of nitro-glycerine. These buildings are—(1) The nitrating house; (2) the separating house; (3) the filter house; (4) the secondary separator; (5) the deposit of washings; (6) the settling or precipitation house; and each of these buildings must be on a level lower than the preceding one, in order that the nitro-glycerine or acids may flow easily from one building to the next. These buildings are, as far as possible, best placed together, and away from the other danger buildings, such as the cartridge huts and dynamite mixing houses, but this is not essential.

All danger buildings should be protected by a lightning conductor, or covered with barbed wire, as suggested by Professor Sir Oliver J. Lodge, F.R.S., Professors Zenger, of Prague, and Melsens, of Brussels, and everything possible should be done to keep them as cool as possible in the summer. With this object they should be made double, and the intervening space filled with cinders. The roof also should be kept whitewashed, and the windows painted over thinly with white paint. A thermometer should be suspended in every house. It is very essential that the floors of all these buildings should be washed every day before the work-people leave. In case any nitro-glycerine is spilt upon the floors, after sponging it up as far as possible, the floor should be washed with an alcoholic solution of soda or potash to decompose the nitro-glycerine, which it does according to the equation[A]—

C{3}H{5}(NO{3}){3} + 3KOH = C{3}H{8}O{3} + 3KNO{3}.

[Footnote A: See also Berthelot, Comptes Rendus, 1900, 131[12], 519- 521.]

Every one employed in the buildings should wear list or sewn leather shoes, which of course must be worn in the buildings only. The various houses should be connected by paths laid with cinders, or boarded with planks, and any loose sand about the site of the works should be covered over with turf or cinders, to prevent its blowing about and getting into the buildings. It is also of importance that stand pipes should be placed about the works with a good pressure of water, the necessary hose being kept in certain known places where they can be at once got at in the case of fire, such as the danger area laboratory, the foreman's office, &c. It is also desirable that the above precautions against fire should be tested once a week. With regard to the heating of the various buildings in the winter, steam pipes only should be used, and should be brought from a boiler-house outside the danger area, and should be covered with kieselguhr or fossil meal and tarred canvas. These pipes may be supported upon poles. A stove of some kind should be placed in the corner of each building, but it must be entirely covered in with woodwork, and as small a length of steam pipes should be within the building as possible.

In the case of a factory where nitro-glycerine and dynamite are manufactured, it is necessary that the work-people should wear different clothes upon the danger area than usual, as they are apt to become impregnated with nitro-glycerine, and thus not very desirable or safe to wear outside the works. It is also necessary that these clothes should not contain any pockets, as this lessens the chance of matches or steel implements being taken upon the danger area. Changing houses, one for the men, and another for the girls, should also be provided. The tools used upon the danger area should, whenever the building is in use, or contains explosives, be made of phosphor bronze or brass, and brass nails or wooden pegs should be used in the construction of all the buildings.

Lightning Conductors.—The Explosive Substances Act, 38 Vict. ch. 17, clause 10, says, "Every factory magazine and expense magazine in a factory, and every danger building in a magazine, shall have attached thereto a sufficient lightning conductor, unless by reason of the construction by excavation or the position of such magazine or building, or otherwise, the Secretary of State considers a conductor unnecessary, and every danger building in a factory shall, if so required by the Secretary of State, have attached thereto a sufficient lightning conductor."

The exact form of lightning conductor most suitable for explosive works and buildings has not yet been definitely settled. Lightning-rod engineers favour what is known as the Melsens system, due to Professor Melsens, of Brussels, and Professor Zenger, of Prague, but first suggested by the late Professor Clerk-Maxwell. In a paper read before the British Association, Clerk-Maxwell proposed to protect powder-magazines from the effects of lightning by completely surrounding or encasing them with sheet metal, or a cage of metallic conductors. There were, however, several objections to his system as he left it.

Professor Melsens[A] has, while using the idea, made several important alterations. He has multiplied the terminals, the conductors, and the earth-connections. His terminals are very numerous, and assume the form of an aigrette or brush with five or seven points, the central point being a little higher than the rest, which form with it an angle of 45 deg.. He employs for the most part galvanised-iron wire. He places all metallic bodies, if they are of any considerable size, in communication with the conducting system in such a manner as to form closed metallic circuits. His system is illustrated in Fig. 2, taken from Arms and Explosives.

[Footnote A: Belgian Academy of Science.]

This system is a near approximation to J.C. Maxwell's cage. The system was really designed for the protection of powder-magazines or store buildings placed in very exposed situations. Zenger's system is identical with that of Melsens, and has been extensively tried by the Austrian military authorities, and Colonel Hess has reported upon the absolute safety of the system.

The French system of protecting powder-magazines is shown in Fig. 3, where there are no brush terminals or aigrettes. The French military authorities also protect magazines by erecting two or more lightning-rods on poles of sufficient height placed close to, but not touching, the walls of the magazine. These conductors are joined below the foundations and earthed as usual.

In the instructions issued by the Government, it is stated that the lightning-rods placed upon powder-mills should be of such a height, and so situated, that no danger is incurred in igniting the powder-dust in the air by the lightning discharge at the pointed rod. In such a case a fork or aigrette of five or more points should invariably be used in place of a single point.

In Fig. 4 (a and b) is shown the Government method for protecting buildings in which explosives are made or stored. Multiple points or aigrettes would be better. Lord Kelvin and Professor Melsens favour points, and it is generally admitted that lightning does not strike buildings at a single point, but rather in a sheet; hence, in such cases, or in the event of the globular form being assumed by the lightning, the aigrette will constitute a much more effective protection than a single point. As to the spacing of conductors, they may, even on the most important buildings, be spaced at intervals of 50 feet. There will then be no point on the building more than 25 feet from the conductor. This "25-feet rule" can be adhered to with advantage in all overground buildings for explosives.

Underground magazines should, whenever possible, also be protected, because, although less exposed than overground buildings, they frequently contain explosives packed in metal cases, and hence would present a line of smaller electrical resistance than the surrounding earth would offer to the lightning. The conductor should be arranged on the same system as for overground buildings, but be applied to the surface of the ground over the magazines.

In all situations where several conductors are joined in one system, the vertical conductors should be connected both at the top and near the ground line. The angles and the prominent portions of a building being the most liable to be struck, the conductors should be carried over and along these projections, and therefore along the ridges of the roof. The conductors should be connected to any outside metal on the roofs and walls, and specially to the foot of rain-water pipes.

All the lightning conductors should be periodically tested, to see that they are in working condition, at least every three months, according to Mr Richard Anderson. The object of the test is to determine the resistance of the earth-connection, and to localise any defective joints or parts in the conductors. The best system of testing the conductors is to balance the resistance of each of the earths against the remainder of the system, from which the state of the earths may be inferred with sufficient accuracy for all practical purposes.

Captain Bucknill, R.E., has designed an instrument to test resistance which is based on the Post Office pattern resistance coil, and is capable of testing to approximate accuracy up to 200 ohms, and to measure roughly up to 2,000 ohms. Mr R. Anderson's apparatus is also very handy, consisting of a case containing three Leclanche cells, and a galvanometer with a "tangent" scale and certain standard resistances. Some useful articles on the protection of buildings from lightning will be found in Arms and Explosives, July, August, and September 1892, and by Mr Anderson, Brit. Assoc., 1878-80.

Nitro-Glycerine.—One of the most powerful of modern explosive agents is nitro-glycerine. It is the explosive contained in dynamite, and forms the greater part of the various forms of blasting gelatines, such as gelatine dynamite and gelignite, both of which substances consist of a mixture of gun-cotton dissolved in nitro-glycerine, with the addition of varying proportions of wood-pulp and saltpetre, the latter substances acting as absorbing materials for the viscid gelatine. Nitro-glycerine is also largely used in the manufacture of smokeless powders, such as cordite, ballistite, and several others.

Nitro-glycerol, or glycerol tri-nitrate, was discovered by Sobrero in the year 1847. In a letter written to M. Pelouse, he says, "when glycerol is poured into a mixture of sulphuric acid of a specific gravity of 1.84, and of nitric acid of a gravity of 1.5, which has been cooled by a freezing mixture, that an oily liquid is formed." This liquid is nitro-glycerol, or nitro-glycerine, which for some years found no important use in the arts, until the year 1863, when Alfred Nobel first started a factory in Stockholm for its manufacture upon a large scale; but on account of some serious accidents taking place, its use did not become general.

It was not until Nobel conceived the idea (in 1866) of absorbing the liquid in some absorbent earth, and thus forming the material that is now known as dynamite, that the use of nitro-glycerine as an explosive became general.

Among those who improved the manufacture of nitro-glycerine was Mowbray, who, by using pure glycerine and nitric acid free from nitrous acid, made very great advances in the manufacture. Mowbray was probably the first to use compressed air for the purpose of keeping the liquids well agitated during the process of nitration, which he conducted in earthenware pots, each containing a charge of 17 lbs. of the mixed acids and 2 lbs. of glycerol.

A few years later (1872), MM. Boutnny and Faucher, of Vonges,[A] proposed to prepare nitro-glycerine by mixing the sulphuric acid with the glycerine, thus forming a sulpho-glyceric acid, which was afterwards mixed with a mixture of nitric and sulphuric acids. They claimed for this method of procedure that the final temperature is much lower. The two mixtures are mixed in the proportions—Glycerine, 100; nitric acid, 280; and sulphuric acid, 600. They state that the rise of temperature upon mixing is limited from 10 deg. to 15 deg. C.; but this method requires a period of twenty-four hours to complete the nitration, which, considering the danger of keeping the nitro-glycerine in contact with the mixed acids for so long, probably more than compensates for the somewhat doubtful advantage of being able to perform the nitration at such a low temperature. The Boutnny process was in operation for some time at Pembrey Burrows in Wales, but after a serious explosion the process was abandoned.

[Footnote A: Comptes Rendus, 75; and Desortiaux, "Traite sur la Poudre," 684-686.]

Nitro-glycerine is now generally made by adding the glycerine to a mixture of sulphuric and nitric acids. The sulphuric acid, however, takes no part in the reaction, but is absolutely necessary to combine with the water that is formed by the decomposition, and thus to keep up the strength of the nitric acid, otherwise lower nitrates of glycerine would be formed that are soluble in water, and which would be lost in the subsequent process of washing to which the nitro-compound is subjected, in order to remove the excess of acids, the retention of which in the nitro-glycerol is very dangerous. Nitro-glycerol, which was formerly considered to be a nitro-substitution compound of glycerol, was thought to be formed thus—

C_{3}H_{8}O_{3} + 3HNO_{3} = C{3}H_{5}(NO_{2})_{3}O_{3} + 3H_{2}O;

but more recent researches rather point to its being regarded as a nitric ether of glycerol, or glycerine, and to its being formed thus—

C{3}H{8}O{3} + 3 HNO{3} = C{3}H{5}(NO{3}){3} + 3H{2}O. 92 227

OH The formula of glycerine is C_{3}H_{8}O_{8}, or C_{3}H_{5} OH OH

ONO_{2} and that of the mono-nitrate of glycerine, C_{3}H_{5} OH OH

ONO_{2} and of the tri-nitrate or (nitro-glycerine), C_{3}H_{5} ONO_{2} ONO_{2}

that is, the three hydrogens of the semi-molecules of hydroxyl in the glycerine have been replaced by the NO_{2} group.

In the manufacture upon the large scale, a mixture of three parts by weight of nitric acid and five parts of sulphuric acid are used. From the above equation it will be seen that every 1 lb. of glycerol should give 2.47 lbs. of nitro-glycerol ((227+1)/92 = 2.47), but in practice the yield is only about 2 lbs. to 2.22, the loss being accounted for by the unavoidable formation of some of the lower nitrate, which dissolves in water, and is thus washed away, and partly perhaps to the presence of a little water (or other non-nitrable matter) in the glycerine, but chiefly to the former, which is due to the acids having become too weak.



Properties of Nitro-Glycerine—Manufacture of Nitro-Glycerine—Nitration— The Nathan Nitrator—Separation—Filtering and Washing—The Waste Acids— Treatment of the Waste Acid from the Manufacture of Nitro-Glycerine and Gun-Cotton.

Properties of Nitro-Glycerine.—Nitro-glycerol is a heavy oily liquid of specific gravity 1.6 at 15 deg. C., and when quite pure is colourless. The commercial product is a pale straw yellow, but varies much according to the purity of the materials used in its manufacture. It is insoluble in water, crystallises at 10.5 deg. C., but different commercial samples behave very differently in this respect, and minute impurities prevent or delay crystallisation. Solid nitro-glycerol[A] melts at about 12 deg. C., but requires to be exposed to this temperature for some time before melting. The specific gravity of the solid form is 1.735 at +10 deg. C.; it contracts one-twelfth of its volume in solidifying. Beckerheim[B] gives the specific heat as 0.4248 between the temperatures of 9.5 deg. and 9.8 deg. C., and L. de Bruyn gives the boiling point as above 200 deg..

[Footnote A: Di-nitro-mono chlorhydrin, when added to nitro-glycerine up to 20 per cent., is said to prevent its freezing.]

[Footnote B: Isb., Chem. Tech., 22, 481-487. 1876.]

Nitro-glycerine has a sweet taste, and causes great depression and vertigo. It is soluble in ether, chloroform, benzene, glacial acetic acid, and nitro-benzene, in 1.75 part of methylated spirit, very nearly insoluble in water, and practically insoluble in carbon bisulphide. Its formula is C{3}H{5}(NO{3}){3}, and molecular weight 227. When pure, it may be kept any length of time without decomposition. Berthelot kept a sample for ten years, and Mr G. M'Roberts, of the Ardeer Factory, for nine years, without their showing signs of decomposition; but if it should contain the smallest trace of free acid, decomposition is certain to be started before long. This will generally show itself by the formation of little green spots in the gelatine compounds, or a green ring upon the surface of liquid nitro-glycerine. Sunlight will often cause it to explode; in fact, a bucket containing some water that had been used to wash nitro-glycerine, and had been left standing in the sun, has in our experience been known to explode with considerable force. Nitro-glycerine when pure is quite stable at ordinary temperatures, and samples have been kept for years without any trace of decomposition. It is very susceptible to heat, and even when quite pure will not stand a temperature of 100 deg. C. for a longer period than a few hours, without undergoing decomposition. Up to a temperature of 45 deg. C., however, properly made and purified nitro- glycerine will remain unchanged almost indefinitely. The percentage composition of nitroglycerine is as follows:—

Found. Theory for C{3}H{5}(N0{2}){3}.

Carbon 15.62 15.86 per cent. Hydrogen 2.40 2.20 " Nitrogen 17.90 18.50 " Oxygen ... 63.44 "

The above analysis is by Beckerheim. Sauer and Adou give the nitrogen as 18.35 to 10.54 per cent. by Dumas' method; but I have never found any difficulty in obtaining percentages as high as 18.46 by the use of Lunge's nitrometer. The decomposition products by explosion are shown by the following equation—

2C{3}H{5}(NO{3}){3} = 6CO{2} + 5H{2}O + 6N + O;

that is, it contains an excess of 3.52 per cent. of oxygen above that required for complete combustion; 100 grms. would be converted into—

Carbonic Acid (CO_{2}) 58.15 per cent. Water 19.83 " Oxygen 3.52 per cent. Nitrogen 18.50 "

The volume of gases produced at 0 deg. and 760 mm., calculated from the above, is 714 litres per kilo, the water being taken as gaseous. Nitro-glycerine is decomposed differently if it is ignited as dynamite (i.e., kieselguhr dynamite), and if the gases are allowed to escape freely under a pressure nearly equal to that of the atmosphere. Sarrau and Vieille obtained under these conditions, for 100 volumes of gas—

NO 48.2 per cent. CO 35.9 " CO{2} 12.7 " H 1.6 per cent. N 1.3 " CH{4} 0.3 "

These conditions are similar to those under which a mining charge, simply ignited by the cap, burns away slowly under a low pressure (i.e., a miss fire). In a recent communication, P.F. Chalon (Engineering and Mining Journal, 1892) says, that in practice nitro-glycerine vapour, carbon monoxide, and nitrous oxide, are also produced as the result of detonation, but he attributes their formation to the use of a too feeble detonator.

Nitro-glycerine explodes very violently by concussion. It may be burned in an open vessel, but if heated above 250 deg. C. it explodes. Professor C.E. Munroe gives the firing point as 2O3 deg.-2O5 deg. C., and L. de Bruyn[A] states its boiling point as 185 deg.. He used the apparatus devised by Horsley. The heat of formation of nitro-glycerine, as deduced from the heat of combustion by M. Longuinine, is 432 calories for 1 grm.; and the heat of combustion equals 1,576 cals. for 1 grm. In the case of nitro-glycerine the heat of total combustion and the heat of complete decomposition are interchangeable terms, since it contains an excess of oxygen. According to Dr W.H. Perkin, F.R.S.,[B] the magnetic rotation of nitro-gylcerine is 5,407, and that of tri-methylene nitrate, 4.769 (diff. = .638). Dr Perkin says: "Had nitro-glycerine contained its nitrogen in any other combination with oxygen than as -O-NO{2}, as it might if its constitution had been represented as C{3}H{2}(NO{2}){3}(OH){3}, the rotation when compared with propyl nitrate (4.085) would be abnormal."

[Footnote A: Jour. Soc. Chem. Ind., June 1896, p. 471.]

[Footnote B: Jour. Chem. Soc., W.H. Perkin, 1889, p. 726.]

The solubility of nitro-glycerine in various solvents has been investigated by A.H. Elliot; his results may be summarised as follows:—

Solvent. Cold. Warm. Water Insoluble Slightly soluble Alcohol, absolute Soluble Soluble " 93% " " " 80% Slowly soluble " " 50% Insoluble Slightly soluble Methyl alcohol Soluble Soluble Amyl " " " Ether, ethylic " " " acetic " " Chloroform " " Acetone " " Sulphuric acid (1.845) " " Nitric acid (1.400) Slowly soluble " Hydrochloric acid (1.200) Insoluble, decomposed Slowly soluble Acetic acid, glacial Soluble Soluble Carbolic acid " " Astral oil Insoluble Insoluble Olive " Soluble Soluble Stearine oil " " Mineral jelly Insoluble Insoluble Glycerine " " Benzene Soluble Soluble Nitro-benzene " " Toluene " " Carbon bi-sulphide Insoluble Slightly affected Turpentine " Soluble Petroleum naphtha, 71 deg.-76 deg. B. " Insoluble Caustic soda (1:10 solution) Insoluble. Insoluble. Borax, 5% solution " " Ammonia (.980) " " slightly affected. Ammonium sulph-hydrate Insoluble, sulphur Decomposed. separates Iron sulphate solution Slightly affected Affected. Iron chloride (1.4 grm. Fe Slowly affected Decomposed. to 10 c.c. N{2}O) Tin chloride Slightly affected Affected.

Many attempts have been made to prepare nitro-glycerine explosives capable of withstanding comparatively low temperatures without freezing, but no satisfactory solution of the problem has been found. Among the substances that have been proposed and used with more or less success, are nitro- benzene, nitro-toluene, di-nitro-mono-chlorhydrine, solid nitro derivatives of toluene,[A] are stated to lower the freezing point of nitro-glycerine to -20 deg.C. without altering its sensitiveness and stability. The subject has been investigated by S. Nauckhoff,[B] who states that nitroglycerine can be cooled to temperatures (-40 deg. to -50 deg. C.) much below its true freezing point, without solidifying, by the addition of various substances. When cooled by means of a mixture of solid carbon, dioxide, and ether, it sets to a glassy mass, without any perceptible crystallisation. The mass when warmed to 0 deg.C. first rapidly liquefies and then begins to crystallise. The true freezing point of pure nitro- glycerine was found to be 12.3 deg.C. The technical product, owing to the presence of di-nitro-glycerine, freezes at 10.5 deg. C. According to Raoult's law, the lowering of the freezing point caused by m grms. of a substance with the molecular weight M, when dissolved in 100 grms. of the solvent, is expressed by the formula: [Delta] = E(m/M), where E is a constant characteristic for the solvent in question. The value of E for nitro- glycerine was found to be 70.5 when calculated, according to Van't Hoff's formula, from the melting point and the latent heat of fusion of the substance. Determinations of the lowering of the freezing point of nitro- glycerine by additions of benzene, nitro-benzene, di-nitro-benzene, tri- nitro-benzene, p.-nitro-toluene, o.-nitro-toluene, di-nitro-toluene, naphthalene, nitro-naphthalene, di-nitro-naphthalene, ethyl acetate, ethyl nitrate, and methyl alcohol, gave results agreeing fairly well with Raoult's formula, except in the case of methyl alcohol, for which the calculated lowering of the freezing point was greater than that observed, probably owing to the formation of complex molecules in the solution. The results show that, in general, the capacity of a substance to lower the freezing point of nitro-glycerine depends, not upon its freezing point, or its chemical composition or constitution, but upon its molecular weight. Nauckhoff states that a suitable substance for dissolving in nitro- glycerine, in order to lower the freezing point of the latter, must have a relatively low molecular weight, must not appreciably diminish the explosive power and stability of the explosive, and must not be easily volatile at relatively high atmospheric temperatures; it should, if possible, be a solvent of nitro-cellulose, and in every case must not have a prejudicial influence on the gelatinisation of the nitro-cellulose.

[Footnote A: Eng. Pat. 25,797, November 1904.]

[Footnote B: Z. Angew. Chem., 1905, 18, 11-22, 53-60.]

Manufacture of Nitro-Glycerine.—Nitro-glycerine is prepared upon the manufacturing scale by gradually adding glycerine to a mixture of nitric and sulphuric acids of great strength. The mixed acids are contained in a lead vessel, which is kept cool by a stream of water continually passing through worms in the interior of the nitrating vessel, and the glycerine is gradually added in the form of a fine stream from above. The manufacture can be divided into three distinct operations, viz., nitration, separation, and washing, and it will be well to describe these operations in the above order.

Nitration.—The most essential condition of nitrating is the correct composition and strength of the mixed acids. The best proportions have been found to be three parts by weight of nitric acid of a specific gravity 1.525 to 1.530, and containing as small a portion of the oxides of nitrogen as possible, to five parts by weight of sulphuric acid of a specific gravity of 1.840 at 15 deg. C., and about 97 per cent. of mono- hydrate. It is of the very greatest importance that the nitric acid should be as strong as possible. Nothing under a gravity of 1.52 should ever be used even to mix with stronger acid, and the nitration will be proportional to the strength of the acid used, provided the sulphuric acid is also strong enough. It is also of great importance that the oxides of nitrogen should be low, and that they should be kept down to as low as 1 per cent., or even lower. It is also very desirable that the nitric acid should contain as little chlorine as possible. The following is the analysis of a sample of nitric acid, which gave very good results upon the commercial scale:—Specific gravity, 1.525, N_{2}O_{4}, 1.03 per cent.; nitric acid (HNO_{3}), 95.58 per cent.

The amount of real nitric acid (mono-hydrate) and the amount of nitric peroxide present in any sample should always be determined before it is used for nitrating purposes. The specific gravity is not a sufficient guide to the strength of the acid, as an acid having a high gravity, due to some 3 or 4 per cent of nitric oxides in solution, will give very poor nitration results. A tenth normal solution of sodium hydroxide (NaOH), with phenol-phthalein as indicator, will be found the most convenient method of determining the total acid present. The following method will be found to be very rapid and reliable:—Weigh a 100 c.c. flask, containing a few cubic centimetres of distilled water, and then add from a pipette 1 c.c. of the nitric acid to be examined, and reweigh (this gives the weight of acid taken). Now make up to 100 c.c. at 15 deg. C.; shake well, and take out 10 c.c. with a pipette; drain into a small Erlenmeyer flask, and add a little of the phenol-phthalein solution, and titrate with the tenth normal soda solution.

The nitric peroxide can be determined with a solution of potassium permanganate of N/10 strength, thus: Take a small conical flask, containing about 10 c.c. of water, and add from a burette 10 to 16 c.c. of the permanganate solution; then add 2 c.c. of the acid to be tested, and shake gently, and continue to add permanganate solution as long as it is decolourised, and until a faint pink colour is permanent.

Example. N/10 permanganate 3.16 grms. per litre, 1 c.c. = O.0046 grm. N{2}O{4}, 2 c.c. of sample of acid specific gravity 1.52 = 3.04 grms. taken for analysis. Took 20 c.c. permanganate solution, O.0046 x 20 =.092 grm. N{2}O{4}, and (.092 x 100)/3.04 = 3.02 per cent. N{2}O{4}. The specific gravity should be taken with an hydrometer that gives the specific gravity directly, or, if preferred, the 2 c.c. of acid may be weighed.

A very good method of rapidly determining the strength of the sulphuric acid is as follows:—Weigh out in a small weighing bottle, as nearly as possible, 2.45 grms. This is best done by running in 1.33 c.c. of the acid (1.33 x 1.84 = 2.447). Wash into a large Erlenmeyer flask, carefully washing out the bottle, and also the stopper, &c. Add a drop of phenol- phthalein solution and titrate, with a half normal solution of sodium hydrate (use a 100 c.c. burette). Then if 2.45 grms. exactly have been taken, the readings on the burette will equal percentages of H{2}SO{4} (mono-hydrate) if not, calculate thus:—2.444 grms. weighed, required 95.4 c.c. NaOH. Then—

2.444 : 95.4 :: 2.45 : x = 95.64 per cent. H{2}SO{4}.

It has been proposed to free nitric acid from the oxides of nitrogen by blowing compressed air through it, and thus driving the gases in solution out. The acid was contained in a closed lead tank, from which the escaping fumes were conducted into the chimney shaft, and on the bottom of which was a lead pipe, bent in the form of a circle, and pierced with holes, through which the compressed air was made to pass; but the process was not found to be of a very satisfactory nature, and it is certainly better not to allow the formation of these compounds in the manufacture of the acid in the first instance. Another plan, however, is to heat the acid gently, and thus drive out the nitrous gases. Both processes involve loss of nitric acid.

Having obtained nitric and sulphuric acids as pure as possible, the next operation is to mix them. This is best done by weighing the carboys in which the acids are generally stored before the acids are drawn off into them from the condensers, and keeping their weights constantly attached to them by means of a label. It is then a simple matter to weigh off as many carboys of acid as may be required for any number of mixings, and subtract the weights of the carboys. The two acids should, after being weighed, be poured into a tank and mixed, and subsequently allowed to flow into an acid egg or montjus, to be afterwards forced up to the nitrating house in the danger area. The montjus or acid egg is a strong cast-iron tank, of either an egg shape, or a cylinder with a round end. If of the former shape, it would lie on its side, and upon the surface of the ground, and would have a manhole at one end, upon which a lid would be strongly bolted down; but if of the latter shape, the lid, of course, is upon the top, and the montjus itself is let into the ground. In either case, the principle is the same. One pipe, made of stout lead, goes to the bottom, and another just inside to convey the compressed air, the acids flowing away as the pressure is put on, just as blowing down one tube of an ordinary wash- bottle forces the water up the other tube to the jet. The pressure necessarily will, of course, vary immensely, and will depend upon the height to which the acid has to be raised and the distance to be traversed.

The mixed acids having been forced up to the danger area, and to a level higher than the position of the nitrating house, should, before being used, be allowed to cool, and leaden tanks of sufficient capacity to hold at least enough acid for four or five nitrations should be placed in a wooden house upon a level at least 6 or 7 feet above the nitrating house. In this house also should be a smaller lead tank, holding, when filled to a certain mark, just enough of the mixed acids for one nitration. The object of this tank is, that as soon as the man in charge knows that the last nitration is finished, he refills this smaller tank (which contains just enough of the mixed acids), and allows its contents to flow down into the nitrating house and into the nitrator, ready for the next nitration. The nitration is usually conducted in a vessel constructed of lead, some 4 feet wide at the bottom, and rather less at the top, and about 4 feet or so high. The size, of course, depends upon the volume of the charge it is intended to nitrate at one operation, but it is always better that the tank should be only two-thirds full. A good charge is 16 cwt. of the mixed acids, in the proportion of three to five; that is, 6 cwt. of nitric acid, and 10 cwt. of sulphuric acid, and 247 lbs. of glycerine.

Upon reference to the equation showing the formation of nitro-glycerine, it will be seen that for every 1 lb. of glycerine 2.47 lbs. of nitro- glycerine should be furnished,[A] but in practice the yield is only a little over 2 lbs., the loss being accounted for by the unavoidable formation of some of the lower nitrate of glycerine (the mono-nitrate), which afterward dissolves in the washing waters. The lead tank (Fig. 5) is generally cased in woodwork, with a platform in front for the man in charge of the nitrating to stand upon, and whence to work the various taps. The top of the tank is closed in with a dome of lead, in which is a small glass window, through which the progress of the nitrating operation can be watched. From the top of this dome is a tube of lead which is carried up through the roof of the building. It serves as a chimney to carry off the acid fumes which are given off during the nitration. The interior of this tank contains at least three concentric spirals of at least 1-inch lead pipe, through which water can be made to flow during the whole operation of nitrating. Another lead pipe is carried through the dome of the tank, as far as the bottom, where it is bent round in the form of a circle. Through this pipe, which is pierced with small holes, about 1 inch apart, compressed air is forced at a pressure of about 60 lbs. in order to keep the liquids in a state of constant agitation during the whole period of nitration. There must also be a rather wide pipe, of say 2 inches internal diameter, carried through the dome of the tank, which will serve to carry the mixed acid to be used in the operation into the tank. There is still another pipe to go through the dome, viz., one to carry the glycerine into the tank. This need not be a large bore pipe, as the glycerine is generally added to the mixed acids in a thin stream (an injector is often used).

[Footnote A: Thus if 92 lbs. glycerine give 227 lbs. nitro-glycerine, (277 x 1)/92 = 2.47 lbs.]

Before the apparatus is ready for use, it requires to have two thermometers fixed, one long one to reach to the bottom of the tank, and one short one just long enough to dip under the surface of the acids. When the tank contains its charge, the former gives the temperature of the bottom, and the latter of the top of the mixture. The glycerine should be contained in a small cistern, fixed in some convenient spot upon the wall of the nitrating house, and should have a pipe let in flush with the bottom, and going through the dome of the nitrating apparatus. It must of course be provided with a tap or stop-cock, which should be placed just above the point where the pipe goes through the lead dome.

Some method of measuring the quantity of glycerine used must be adopted. A gauge-tube graduated in inches is a very good plan, but it is essential that the graduations should be clearly visible to the operator upon the platform in front of the apparatus. A large tap made of earthenware (and covered with lead) is fixed in the side of the nitrating tank just above the bottom, to run off the charge after nitration. This should be so arranged that the charge may be at option run down the conduit to the next house or discharged into a drowning tank, which may sometimes be necessary in cases of decomposition. The drowning tank is generally some 3 or 4 yards long and several feet deep, lined with cement, and placed close outside the building.

The apparatus having received a charge of mixed acids, the water is started running through the pipes coiled inside the tank, and a slight pressure of compressed air is turned on,[A] to mix the acids up well before starting. The nitration should not be commenced until the two thermometers register a temperature of 18 deg. C. The glycerine tap is then partially opened, and the glycerine slowly admitted, and the compressed air turned on full, until the contents of the apparatus are in a state of very brisk agitation. A pressure of about 40 lbs. is about the minimum (if 247 lbs. of glycerine and 16 cwt. of acids are in the tank). If the glycerine tube is fitted with an injector, it may be turned on almost at once. The nitration will take about thirty minutes to complete, but the compressed air and water should be kept on for an additional ten minutes after this, to give time for all the glycerine to nitrate. The temperature should be kept as low as possible (not above 18 deg. C.).

[Footnote A: At the Halton Factory, Germany, cylinders of compressed carbon dioxide are connected with the air pipes so that in the event of a failure of the air supply the stirring can be continued with this gas if necessary.]

The chief points to attend to during the progress of the nitration are—

1. The temperature registered by the two thermometers.

2. The colour of the nitrous fumes given off (as seen through the little window in the dome of the apparatus).

3. The pressure of the compressed air as seen from a gauge fixed upon the air pipe just before it enters the apparatus.

4. The gauge showing the quantity of glycerine used. The temperature, as shown by either of the two thermometers, should not be at any time higher than 25 deg. C.

If it rises much above this point, the glycerine should be at once shut off, and the pressure of air increased for some few minutes until the temperature falls, and no more red fumes are given off.

The nitration being finished, the large earthenware tap at the bottom of the tank is opened, and the charge allowed to flow away down the conduit to the next building, i.e., to the separator.

The nitrating house is best built of wood, and should have a close-boarded floor, which should be kept scrupulously clean, and free from grit and sand. A wooden pail and a sponge should be kept in the house in order that the workman may at once clean up any mess that may be made, and a small broom should be handy, in order that any sand, &c., may be at once removed. It is a good plan for the nitrator to keep a book in which he records the time of starting each nitration, the temperature at starting and at the finish, the time occupied, and the date and number of the charge, as this enables the foreman of the danger area at any time to see how many charges have been nitrated, and gives him other useful information conducive to safe working. Edward Liebert has devised an improvement in the treatment of nitro-glycerine. He adds ammonium sulphate or ammonium nitrate to the mixed acids during the operation of nitrating, which he claims destroys the nitrous acid formed according to the equation—

(NH{4}){2}SO{4} + 2HNO{3} = H{2}SO{4} + 2N{2} + 4H{2}O.

I am not aware that this modification of the process of nitration is in use at the present time.

The newly made charge of nitro-glycerine, upon leaving the nitrating house, flows away down the conduit, either made of rubber pipes, or better still, of woodwork, lined with lead and covered with lids made of wood (in short lengths), in order that by lifting them at any point the condition of the conduit can be examined, as this is of the greatest importance, and the conduit requires to be frequently washed out and the sulphate of lead removed. This sulphate always contains nitro-glycerine, and should therefore be burnt in some spot far removed from any danger building or magazine, as it frequently explodes with considerable violence.

In works where the manufacture of nitro-glycerine is of secondary importance, and some explosive containing only perhaps 10 per cent. of nitroglycerine is manufactured, and where 50 or 100 lbs. of glycerine are nitrated at one time, a very much smaller nitrating apparatus than the one that has been already described will be probably all that is required. In this case the form of apparatus shown in Fig. 6 will be found very satisfactory. It should be made of stout lead (all lead used for tanks, &c., must be "chemical lead"), and may be made to hold 50 or 100 lbs. as found most convenient. This nitrator can very well be placed in the same house as the separator; in fact, where such a small quantity of nitro- glycerine is required, the whole series of operations, nitrating, separation, and washing, &c., may very well be performed in the same building. It will of course be necessary to place the nitrator on a higher level than the separator, but this can easily be done by having platforms of different heights, the nitration being performed upon the highest. The construction of this nitrator is essentially the same as in the larger one, the shape only being somewhat different. Two water coils will probably be enough, and one thermometer. It will not be necessary to cover this form in with woodwork.

The Nathan Nitrator.[A]—This nitrator is the patent of Lt. Col. F.L. Nathan and Messrs J.M. Thomson and W. Rintoul of Waltham Abbey, and will probably before long entirely supersede all the other forms of nitrator on account of its efficiency and economy of working. With this nitrator it is possible to obtain from 2.21 to 2.22 parts of nitro-glycerine from every 1 part of glycerine. The apparatus is so arranged that the nitration of the glycerine, the separation of nitro-glycerine produced, as well as the operation of "after-separation," are carried out in one vessel. The usual nitrating vessel is provided with an acid inlet pipe at the bottom, and a glass separation cylinder with a lateral exit or overflow pipe at the top. This cylinder is covered by a glass hood or bell jar during nitration to direct the escaping air and fumes into a fume pipe where the flow of the latter may be assisted by an air injector. The lateral pipe in the separation cylinder is in connection with a funnel leading to the prewash tank. The drawing (Fig. 7) shows a vertical section of the apparatus; a is the nitrating vessel of usual construction, having at the bottom an acid inlet pipe with three branches, one leading to the de-nitrating plant, c leading to the drowning tank, and d, which extends upwards and has two branches, e leading to the nitrating acids tank, and f to the waste acid tank. On the sloped bottom of the nitrating vessel a lies a coil g of perforated pipe for blowing air, and there are in the vessel several coils h, three shown in the drawing, for circulation of cooling water. At the top of the vessel there is a glass cylinder i, having a lateral outlet j directed into the funnel mouth of a pipe k leading to the prewash tank. Over the cylinder i is a glass globe l, into which opens a pipe m for leading off fumes which may be promoted by a compressed air jet from a pipe r operating as an injector. Into an opening of the glass dome l is inserted a vessel n, which is connected by a flexible pipe p to the glycerine tank, and from the bottom of n, which is perforated and covered with a disc perforated with holes registering with those through the bottom, this disc being connected by a stem with a knob q by which it can be turned so as to throttle or cut off passage of glycerine through the bottom. s is a thermometer for indicating the temperature of the contents of the vessel.

[Footnote A: Eng. Pat. 15,983, August 1901.]

In operating with this apparatus the nitrating acid is introduced into the nitrating vessel by opening the cock of the pipe e. The glycerine is then run in by introducing n and opening the valve at its bottom, the contents of the vessel being agitated by air blown through the perforations of the pipe g. When the glycerine is all nitrated and the temperature has slightly fallen, the circulation of the water through the coils h and the air-stirring are stopped, and the glycerine supply vessel n is removed. The nitro-glycerine as it separates from the acids is raised by introducing by the pipe f waste acid from a previous charge, this displacing the nitro-glycerine upwards and causing it to flow by the outlet, j and pipe k to the prewash tank. When nearly all the nitro-glycerine has been separated in this manner the acids in the apparatus may be run off by the pipe b to an after separating vessel for further settling, thus leaving the apparatus free for another nitration, or the nitrating vessel itself may be used as an after separating bottle displacing the nitro-glycerine with waste acid as it rises to the top, or skimming off in the usual manner. When the separation of the nitro- glycerine is complete the waste acid is run off and denitrated as usual, a portion of it being reserved for the displacement of the nitro-glycerine in a subsequent operation.

In a further patent (Eng. Pat. 3,020, 1903) the authors propose with the object of preventing the formation and separation of nitro-glycerine in the waste acids, after the nitro-glycerine initially formed in the nitrating vessel has been separated and removed, to add a small quantity of water to the waste acids; this is carried out as follows. A relatively small quantity of water is added, and this prevents all further separation of nitro-glycerine, and at the same time the strength of the waste acids is so slightly reduced that their separation and re-concentration are not affected. "After-separation" is thus done away with, and the nitro- glycerine plant simplified and its output increased. After nitration separation is commenced at a temperature such that when all the displacing acid has been added, and the separation of the nitro-glycerine is complete, the temperature of the contents of the nitrating vessel shall not be lower than 15 deg. C. A sufficient quantity of the displacing acid is then run off through the waste-acid cock to allow of the remaining acids being air-stirred without splashing over the top. A small quantity of water, from 2 to 3 per cent. according to strength of acid; if waste consists of sulphuric acid (monohydrate), 62 per cent.; nitric acid (anhydrous), 33 per cent. and water 5 per cent.; temperature 15 deg. C., then 2 per cent. of water is added; if waste acids contain less than 4 per cent. of water of temperature lower than 15 deg. C., from 3 to 5 per cent. of water may have to be added. The water is added slowly through the separator cylinder, and the contents of the nitrator air-stirred, but not cooled, the temperature being allowed to rise slowly and regularly as the water is added—usually about 3 deg. C. for each per cent. of water added. When air-agitation has been stopped, the acids are kept at rest for a short time, in order to allow of any small quantity of initially formed nitro-glycerine adhering to the coils and sides of the vessel rising to the top. When this has been separated by displacement, the acids are ready for denitration, or can be safely stored without further precaution.

Separation.—The nitro-glycerine, together with the mixed acids, flows from the nitrating house to the separating house, which must be on a lower level than the former. The separating house contains a large lead-lined tank, closed in at the top with a wooden lid, into which a lead pipe of large bore is fixed, and which is carried up through the roof of the building, and acts as a chimney to carry off any fumes. A little glass window should be fixed in this pipe in order that the colour of the escaping fumes may be seen. The conduit conveying the nitro-glycerine enters the building close under the roof, and discharges its contents into the tank through the pipe G (Fig. 8). The tank is only about two-thirds filled by the charge. There is in the side of the tank a small window of thick plate glass, which enables the workman to see the level of the charge, and also to observe the progress of the separation, which will take from thirty minutes to one hour.

The tank should be in connection with a drowning tank, as the charge sometimes gets very dangerous in this building. It must also be connected by a conduit with the filter house, and also to the secondary separator by another conduit. The tank should also be fitted with a compressed air pipe, bent in the form of a loop. It should lie upon the bottom of the vat. The object of this is to mix up the charge in case it should get too hot through decomposition. A thermometer should of course be fixed in the lid of the tank, and its bulb should reach down to the middle of the nitro-glycerine (which rests upon the surface of the mixed acids, the specific gravity of the nitro-glycerine being 1.6, and that of the waste acids 1.7; the composition of the acids is now 11 per cent. HNO_{3}, 67 per cent. H_{2}SO_{4}, and 22 per cent. water), and the temperature carefully watched.

If nothing unusual occurs, and it has not been necessary to bring the compressed air into use, and so disturb the process of separation, the waste acids may be run away from beneath the nitro-glycerine, and allowed to flow away to the secondary separator, where any further quantity of nitro-glycerine that they contain separates out after resting for some days. The nitro-glycerine itself is run into a smaller tank in the same house, where it is washed three or four times with its own bulk of water, containing about 3 lbs. of carbonate of soda to neutralise the remaining acid. This smaller tank should contain a lead pipe, pierced and coiled upon the bottom, through which compressed air may be passed, in order to stir up the charge with the water and soda. After this preliminary washing, the nitro-glycerine is drawn off into indiarubber buckets, and poured down the conduit to the filter house. The wash waters may be sent down a conduit to another building, in order to allow the small quantity of nitro-glycerine that has been retained in the water as minute globules to settle, if thought worth the trouble of saving. This, of course, will depend upon the usual out-turn of nitro-glycerine in a day, and the general scale of operations.

Filtering and Washing.—The filter house (Fig. 9), which must of course be again on a somewhat lower level than the separating house, must be a considerably larger building than either the nitrating or separating houses, as it is always necessary to be washing some five or six charges at the same time. Upon the arrival of the nitro-glycerine at this house, it first flows into a lead-lined wooden tank (W), containing a compressed air pipe, just like the one in the small tank in the separating house. This tank is half filled with water, and the compressed air is turned on from half to a quarter of an hour after the introduction of the charge. The water is then drawn off, and fresh water added. Four or five washings are generally necessary. The nitro-glycerine is then run into the next tank (A), the top of which is on a level with the bottom of the first one. Across the top of this tank is stretched a frame of flannel, through which the nitroglycerine has to filter. This removes any solid matters, such as dirt or scum. Upon leaving this tank, it passes through a similar flannel frame across another tank (B), and is finally drawn off by a tap in the bottom of the tank into rubber buckets. The taps in these tanks are best made of vulcanite.

At this stage, a sample should be taken to the laboratory and tested. If the sample will not pass the tests, which is often the case, the charge must be rewashed for one hour, or some other time, according to the judgment of the chemist in charge. In the case of an obstinate charge, it is of much more avail to wash a large number of times with small quantities of water, and for a short time, than to use a lot of water and wash for half an hour. Plenty of compressed air should be used, as the compound nitric ethers which are formed are thus got rid of. As five or six charges are often in this house at one time, it is necessary to have as many tanks arranged in tiers, otherwise one or two refractory charges would stop the nitrating house and the rest of the nitro-glycerine plant. The chief causes of the washed material not passing the heat test are, either that the acids were not clean, or they contained objectionable impurities, or more frequently, the quality of the glycerine used. The glycerine used for making nitro-glycerine should conform to the following tests, some of which, however, are of greater importance than others. The glycerine should—

1. Have minimum specific gravity at 15 deg. C. of 1.261.

2. Should nitrify well.

3. Separation should be sharp within half an hour, without the separation of flocculent matter, nor should any white flocculent matter (due to fatty acids) be formed when the nitrated glycerine is thrown into water and neutralised with carbonate of soda.

4. Should be free from lime and chlorine, and contain only traces of arsenic, sulphuric acid, &c.

5. Should not leave more than 0.25 per cent. of inorganic and organic residue together when evaporated in a platinum dish without ebullition (about 160 deg. C.) or partial decomposition.

6. Silver test fair.

7. The glycerine, when diluted one-half, should give no deposit or separation of fatty acids when nitric peroxide gas is passed through it. (Nos. 1, 2, 3, and 5 are the most essential.)

The white flocculent matter sometimes formed is a very great nuisance, and any sample of glycerol which gives such a precipitate when tried in the laboratory should at once be rejected, as it will give no end of trouble in the separating house, and also in the filter house, and it will be very difficult indeed to make the nitro-glycerine pass the heat test. The out- turn of nitro-glycerine also will be very low. The trouble will show itself chiefly in the separating operation. Very often 2 or 3 inches will rise to the surface or hang about in the nitro-glycerine, and at the point of contact between it and the mixed acids, and will afterwards be very difficult to get rid of by filtration. The material appears to be partly an emulsion of the glycerine, and partly due to fatty acids, and as there appears to be no really satisfactory method of preventing its formation, or of getting rid of it, the better plan is not to use any glycerine for nitrating that has been found by experiment upon the laboratory scale to give this objectionable matter. One of the most useful methods of testing the glycerine, other than nitrating, is to dilute the sample one-half with water, and then to pass a current of nitric peroxide gas through it, when a flocculent precipitate of elaidic acid (less soluble in glycerine than the original oleic acid) will be formed. Nitrogen peroxide, N{2}O{4}, is best obtained by heating dry lead nitrate (see Allen, "Commercial Organic Analysis," vol. ii., 301).

When a sample of nitro-glycerine is brought to the laboratory from the filter house, it should first be examined to see that it is not acid.[A] A weak solution of Congo red or methyl orange may be used. If it appears to be decidedly alkaline, it should be poured into a separating funnel, and shaken with a little distilled water. This should be repeated, and the washings (about 400 c.c.) run into a beaker, a drop of Congo red or methyl orange added, and a drop or so of N/2 hydrochloric acid added, when it should give, with two or three drops at most, a blue colour with the Congo red, or pink with the methyl orange, &c. The object of this test is to show that the nitro-glycerine is free from any excess of soda, i.e., that the soda has been properly washed out, otherwise the heat test will show the sample to be better than it is. The heat test must also be applied.

[Footnote A: A. Leroux, Bul. Soc. Chim. de Bel., xix., August 1905, contends that experience does not warrant the assumption that free acid is a source of danger in nitro-glycerine or nitro-cellulose; free alkali, he states, promotes their decomposition.]

Upon leaving the filter house, where it has been washed and filtered, and has satisfactorily passed the heat test, it is drawn off from the lowest tank in indiarubber buckets, and poured down the conduit leading to the precipitating house, where it is allowed to stand for a day, or sometimes longer, in order to allow the little water it still contains to rise to the surface. In order to accomplish this, it is sufficient to allow it to stand in covered-in tanks of a conical form, and about 3 or 4 feet high. In many works it is previously filtered through common salt, which of course absorbs the last traces of water. It is then of a pale yellow colour, and should be quite clear, and can be drawn off by means of a tap (of vulcanite), fixed at the bottom of the tanks, into rubber buckets, and is ready for use in the preparation of dynamite, or any of the various forms of gelatine compounds, smokeless powders, &c., such as cordite, ballistite, and many others.

Mikolajezak (Chem. Zeit., 1904, Rep. 174) states that he has prepared mono- and di-nitro-glycerine, and believes that the latter compound will form a valuable basis for explosives, as it is unfreezable. It is stated to be an odourless, unfreezable oil, less sensitive to percussion, friction, and increase of temperature, and to possess a greater solvent power for collodion-cotton than ordinary nitro-glycerine. It can thus be used for the preparation of explosives of high stability, which will maintain their plastic nature even in winter. The di-nitro-glycerine is a solvent for tri-nitro-glycerine, it can therefore be mixed with this substance, in the various gelatine explosives in order to lower the freezing point.

The Waste Acids.—The waste acids from the separating house, from which the nitro-glycerine has been as completely separated as possible, are run down the conduit to the secondary separator, in order to recover the last traces of nitro-glycerine that they contain. The composition of the waste acids is generally somewhat as follows:—Specific gravity, 1.7075 at 15 deg. C.; sulphuric acid, 67.2 per cent.; nitric acid, 11.05 per cent.; and water, 21.7 per cent., with perhaps as much as 2 per cent. of nitric oxide, and of course varying quantities of nitro-glycerine, which must be separated, as it is impossible to run this liquid away (unless it can be run into the sea) or to recover the acids by distillation as long as it contains this substance. The mixture, therefore, is generally run into large circular lead-lined tanks, covered in, and very much like the nitrating apparatus in construction, that is, they contain worms coiled round inside, to allow of water being run through to keep the mixture cool, and a compressed air pipe, in order to agitate the mixture if necessary. The top also should contain a window, in order to allow of the interior being seen, and should have a leaden chimney to carry off the fumes which may arise from decomposition. It is also useful to have a glass tube of 3 or 4 inches in diameter substituted for about a foot of the lead chimney, in order that the man on duty can at any time see the colour of the fumes arising from the liquid. There should also be two thermometers, one long one reaching to the bottom of the tank, and one to just a few inches below the surface of the liquid.

The nitro-glycerine, of course, collects upon the surface, and can be drawn off by a tap placed at a convenient height for the purpose. The cover of the tank is generally conical, and is joined to a glass cylinder, which is cemented to the top of this lead cover, and also to the lead chimney. In this glass cylinder is a hole into which fits a ground glass stopper, through which the nitro-glycerine can be drawn off. There will probably never be more than an inch of nitro-glycerine at the most, and seldom that. It should be taken to the filter house and treated along with another charge. The acids themselves may either be run to waste, or better treated by some denitration plant. This house probably requires more attention than any other in the danger area, on account of the danger of the decomposition of the small quantities of nitro-glycerine, which, as it is mixed with such a large quantity of acids and water, is very apt to become hot, and decomposition, which sets up in spots where a little globule of nitro-glycerine is floating, surrounded by acids that gradually get hot, gives off nitrous fumes, and perhaps explodes, and thus causes the sudden explosion of the whole. The only way to prevent this is for the workman in charge to look at the thermometers frequently, and at the colour of the escaping fumes, and if he should notice a rise of temperature or any appearance of red fumes, to turn on the water and air, and stir up the mixture, when probably the temperature will suddenly fall, and the fumes cease to come off.

The cause of explosions in this building is either the non-attention of the workmen in charge, or the bursting of one of the water pipes, by which means, of course, the water, finding its way into the acids, causes a sudden rise of temperature. If the latter of these two causes should occur, the water should at once be shut off and the air turned on full, but if it is seen that an explosion is likely to occur, the tank should at once be emptied by allowing its contents to run away into a drowning tank placed close outside the house, which should be about 4 feet deep, and some 16 feet long by 6 feet wide; in fact, large enough to hold a considerable quantity of water. But this last course should only be resorted to as a last extremity, as it is extremely troublesome to recover the small quantity of nitro-glycerine from the bottom of this tank, which is generally a bricked and cemented excavation some few yards from the house.

It has been proposed to treat these waste acids, containing nitro- glycerine, in Mr M. Prentice's nitric acid retort. In this case they would be run into the retort, together with nitrate of soda, in a fine stream, and the small quantity of nitro-glycerine, coming into contact with the hot mixture already in the retort, would probably be at once decomposed. This process, although not yet tried, promises to be a success. Several processes have been used for the denitration of these acids.

Treatment of the Waste Acid from the Manufacture of Nitro-Glycerine and Gun-Cotton.—The composition of these acids is as follows:—

Nitro-glycerine and Gun-cotton Waste Acid.

Sulphuric acid 70 per cent. 78 per cent. Nitric acid 10 " 12 " Water 20 " 10 "

The waste acid from the manufacture of gun-cotton is generally used direct for the manufacture of nitric acid, as it contains a fairly large amount of sulphuric acid, and the small amount of nitro-cellulose which it also generally contains decomposes gradually and without explosion in the retort. Nitric acid may be first distilled off, the resulting sulphuric acid being then added to the equivalent amount of nitrate of soda. Nitric acid is then distilled over and condensed in the usual way. Very often, however, the waste acid is added direct to the charge of nitrate without previously eliminating the nitric acid. The treatment of the waste acid from the manufacture of nitro-glycerine is somewhat different. The small amount of nitro-glycerine in this acid must always be eliminated. This is effected either by allowing the waste acid to stand for at least twenty- four hours in a big vessel with a conical top, where all the nitro- glycerine which will have separated to the surface is removed by skimming; or, better still, the "watering down process" of Col. Nathan may be employed. In Nathan's nitrator every existing trace of nitro-glycerine is separated from the acids in a few hours after the nitration, and any further formation of nitro-glycerine is prevented by adding about 2 per cent. of water to the waste acids, which are kept agitated during the addition. The waste acid, now free from nitro-glycerine, but which may still contain organic matter, is denitrated by bringing it into contact with a jet of steam. The waste acid is passed in a small stream down through a tower of acid-resisting stoneware (volvic stone), which is closely packed with earthenware, and at the bottom of which is the steam jet. Decomposition proceeds as the acid meets the steam, nitric and nitrous acids are disengaged and are passed out at the top of the tower through a pipe to a series of condensers and towers, where the nitric acid is collected. The nitrous acid may be converted into nitric acid by introducing a hot compressed air jet into the gases before they pass into the condensers. Weak sulphuric acid of sp. gr. 1.6 collects in a saucer in which the tower stands, and is then passed through a cooling worm. The weak sulphuric acid, now entirely free from nitric and nitrous acids, may be concentrated to sp. gr. 1.842 and 96 per cent. H{2}SO{4} by any of the well-known processes, e.g., Kessler, Webb, Benker, Delplace, &c., and it may be used again in the manufacture of nitro-glycerine or gun-cotton.

Two points in the manufacture of nitro-glycerine are of the greatest importance, viz., the purity of the glycerine used, and the strength and purity of the acids used in the nitration. With regard to the first of these, great care should be taken, and a complete analysis and thorough examination, including a preliminary experimental nitration, should always be instituted. As regards the second, the sulphuric acid should not only be strong (96 per cent.), but as free from impurities as possible. With the nitric acid, which is generally made at the explosive works where it is used, care must be taken that it is as strong as possible (97 per cent. and upwards). This can easily be obtained if the plant designed by Mr Oscar Guttmann[A] is used. Having worked Mr Guttmann's plant for some time, I can testify as to its value and efficiency.

[Footnote A: "The Manufacture of Nitric Acid," Jour. Soc. Chem. Ind., March 1893.]

Another form of nitric acid plant, which promises to be of considerable service to the manufacturer of nitric acid for the purpose of nitrating, is the invention of the late Mr Manning Prentice, of Stowmarket. Through the kindness of Mr Prentice, I visited his works to see the plant in operation. It consists of a still, divided into compartments or chambers in such a manner that the fluid may pass continuously from one to the other. The nitric acid being continuously separated by distillation, the contents of each division vary—the first containing the full proportion of nitric acid, and each succeeding one less of the nitric acid, until from the overflow of the last one the bisulphate of soda flows away without any nitric acid. The nitrate of soda is placed in weighed quantities in the hopper, whence it passes to the feeder. The feeder is a miniature horizontal pug-mill, which receives the streams of sulphuric acid and of nitrate, and after thoroughly mixing them, delivers them into the still, where, under the influence of heat, they rapidly become a homogeneous liquid, from which nitric acid continuously distils.

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