~Di-nitro Benzene~ is obtained by treating a charge of the hydrocarbon benzene with double the quantity of mixed acids in two operations, or rather in two stages, the second lot of acid being run in directly after the first. The cooling water is then shut off, and the temperature allowed to rise rapidly, or nitro-benzene already manufactured is taken and again nitrated with acids. A large quantity of acid fumes come off, and some of the nitro- and di-nitro-benzol produced comes off at the high temperature which is attained, and a good condensing apparatus of stoneware must be used to prevent loss. The product is separated from the acids, washed with cold water and then with hot. It is slightly soluble in water, so that the washing waters must be kept and used over again. Finally it is allowed to settle, and run while still warm into iron trays, in which it solidifies in masses 2 or 4 inches thick. It should not contain any nitro-benzol, nor soil a piece of paper when laid on it, should be well crystallised, fairly hard, and almost odourless. The chief product is meta-di-nitro-benzene, melting point 89.8, but ortho-di-nitro-benzene, melting point 118°, and para-di-nitro, melting point 172°, are also produced. The melting point of the commercial product is between 85° to 87° C.
Di-nitro-toluene is made in a similar manner. The tri-nitro-benzene can only be made by using a very large excess of the mixed acids. Nitro- benzene, when reduced with iron, zinc, or tin, and hydrochloric acids, forms aniline.
~Roburite.~—This explosive is the invention of a German chemist, Dr Carl Roth (English patent 267A, 1887), and is now manufactured in England, at Gathurst, near Wigan. It consists of two component parts, non-explosive in themselves (Sprengel's principle), but which, when mixed, form a powerful explosive. The two substances are ammonium nitrate and chlorinated di-nitro-benzol. Nitro-naphthalene is also used. Nitrate of soda and sulphate of ammonium are allowed to be mixed with it. The advantages claimed for the introduction of chlorine into the nitro compound are that chlorine exerts a loosening effect upon the NO_{2} groups, and enables the compound to burn more rapidly than when the nitro groups alone are present.
The formula of chloro-di-nitro-benzol is C_{6}H_{3}Cl(NO_{2})_{2}. The theoretical percentage of nitrogen, therefore, is 13.82, and of chlorine 17.53. Dr Roth states that, from experiments he has made, the dynamic effect is considerably increased by the introduction of chlorine into the nitro compound. Roburite burns quickly, and is not sensitive to shock; it must be used dry; it cannot be made to explode by concussion, pressure, friction, fire, or lightning; it does not freeze; it does not give off deleterious fumes, and it is to all intents and purposes flameless; and when properly tamped and fired by electricity, can be safely used in fiery mines, neither fine dust nor gases being ignited by it. The action is rending and not pulverising. Compared to gunpowder, it is more powerful in a ratio ranging from 2-1/2 to 4 to 1, according to the substance acted upon. It is largely used in blasting, pit sinking, quarrying, &c., but especially in coal mining. According to Dr Roth, the following is the equation of its decomposition:—
C_{6}H_{3}Cl(NO_{2}){2} + 9HN{4}NO_{3} = 6CO_{2} + 20N + HCl.
In appearance roburite is a brownish yellow powder, with the characteristic smell of nitro-benzol. Its specific gravity is 1.40. The Company's statement that the fumes of roburite were harmless having been questioned by the miners of the Garswood Coal and Iron Works Colliery, a scientific committee was appointed by the management and the men jointly for the purpose of settling the question. The members of this committee were Dr N. Hannah, Dr D.J. Mouncey, and Professor H.B. Dixon, F.R.S., of Owens College. After a protracted investigation, a long and technical report was issued, completely vindicating the innocuousness of roburite when properly used. In the words of The Iron and Coal Trades' Review (May 24, 1889), "The verdict, though not on every point in favour of the use in all circumstances of roburite in coal mines, is yet of so pronounced a character in its favour as an explosive that it is impossible to resist the conclusion that the claims put forward on its behalf rest on solid grounds."
Roburite was also one of the explosives investigated by the committee appointed in September 1889 by the Durham Coalowners' and Miners' Associations, for the purpose of determining whether the fumes produced by certain explosives are injurious to health. Both owners and workmen were represented on the committee, which elected Mr T. Bell, H.M. Inspector of Mines, as its chairman, with Professor P.P. Bedson and Drs Drummond and Hume as professional advisers. The problem considered was whether the fumes produced by the combustion of certain explosives, one of which was roburite, were injurious to health. The trial comprised the chemical analysis of the air at the "intake," and of the vitiated air during the firing of the shots at the "return," and also of the smoky air in the vicinity of the shot-holes. Five pounds and a half of roburite were used in twenty-three shots. It had been asserted that the fumes from this explosive contained carbon-monoxide, CO, but no trace of this gas could be discovered after the explosion. On another occasion, however, when 4.7 lbs. of roburite were exploded in twenty-three shots, the air at the "return" showed traces of CO gas to the extent of .042 to .019 per cent. The medical report which Drs Hume and Drummond presented to the committee shows that they investigated every case of suspected illness produced by exposure to fumes, and they could find no evidence of acute illness being caused. They say, "No case of acute illness has, throughout the inquiry, been brought to our knowledge, and we are led to the conclusion that such cases have not occurred."
~Manufacture.~—As now made, roburite is a mixture of ammonium nitrate and chlorinated di-nitro-benzol. The nitrate of ammonia is first dried and ground, and then heated in a closed steam-jacketed vessel to a temperature of 80° C., and the melted organic compound is added, and the whole stirred until an intimate mixture is obtained. On cooling, the yellow powder is ready for use, and is stored in straight canisters or made up into cartridges. Owing to the deliquescent nature of the nitrate of ammonia, the finished explosive must be kept out of contact with the air, and for this reason the cartridges are waterproofed by dipping them in melted wax. Roburite is made in Germany, at Witten, Westphalia; and also at the English Company's extensive works at Gathurst, near Wigan, which have been at work now for some eighteen years, having started in 1888. These works are of considerable extent, covering 30 acres of ground, and are equal to an output of 10 tons a day. A canal runs through the centre, separating the chemical from the explosive portions of the works, and the Lancashire and Yorkshire Railway runs up to the doors. Besides sending large quantities of roburite itself abroad, the Company also export to the various colonies the two components, as manufactured in the chemical works, and which separately are quite non-explosive, and which, having arrived at their destination, can be easily mixed in the proper proportions.
Among the special advantages claimed for roburite are:—First, that it is impossible to explode a cartridge by percussion, fire, or electric sparks. If a cartridge or layer be struck with a heavy hammer, the portion struck is decomposed, owing to the large amount of heat developed by the blow. The remaining explosive is not in the least affected, and no detonation whatever takes place. If roburite be mixed with gunpowder, and the gunpowder fired, the explosion simply scatters the roburite without affecting it in the least. In fact, the only way to explode roburite is to detonate it by means of a cap of fulminate, containing at least 1 gramme of fulminate of mercury. Secondly, its great safety for use in coal mines. Roburite has the great advantage of exploding by detonation at a very low temperature, indeed so low that a very slight amount of tamping is required when fired in the most explosive mixture of air and coal gas possible, and not at all in a mixture of air and coal dust—a condition in which the use of gunpowder is highly dangerous.
Mr W.J. Orsman, F.I.C., in a paper read at the University College, Nottingham, in 1893, gives the temperature of detonation of roburite as below 2,100° C., and of ammonium nitrate as 1,130° C., whereas that of blasting gelatine is as much as 3,220° C. With regard to the composition of the fumes formed by the explosion of roburite, Mr Orsman says: "With certain safety explosives—roburite, for instance—an excess of the oxidising material is added, namely, nitrate of ammonia; but in this case the excess of oxygen here causes a diminution of temperature, as the nitrate of ammonia on being decomposed absorbs heat. This excess of oxygen effectually prevents the formation of carbon monoxide (CO) and the oxides of nitrogen."