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.
Mr Prentice says: "I may point out that while the ordinary process of making nitric acid is one of fractional distillation by time, mine is fractional distillation by space." "Instead of the operation being always at the same point of space, but differing by the successive points of time, I arrange for the differences to take place at different points of space, and these differences exist at one and the same points of time." It is possible with this plant to produce the full product of nitric acid of a gravity of 1.500, or to obtain the acid of varying strengths from the different still-heads. One of these stills, capable of producing about 4 tons of nitric acid per week, weighs less than 2 tons. It is claimed that there is by their use a saving of more than two-thirds in fuel, and four- fifths in condensing plant. Further particulars and illustrations will be found in Mr Prentice's paper (Journal of the Society of Chemical Industry, 1894, p. 323).
CHAPTER III.
NITRO-CELLULOSE, &c.
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.
~The Nitro-Celluloses.~—The substance known as cellulose forms the groundwork of vegetable tissues. The cellulose of the woody parts of plants was at one time supposed to be a distinct body, and was called lignine, but they are now regarded as identical. The formula of cellulose is (C_{6}H_{10}O_{6}){X}, and it is generally assumed that the molecular formula must be represented by a multiple of the empirical formula, C{12}H_{20}O_{10} being often regarded as the minimum. The assumption is based on the existence of a penta-nitrate and the insoluble and colloidal nature of cellulose. Green (Zeit. Farb. Text. Ind., 1904, 3, 97) considers these reasons insufficient, and prefers to employ the single formula C_{6}H_{10}O_{5}. Cellulose can be extracted in the pure state, from young and tender portions of plants by first crushing them, to rupture the cells, and then extracting with dilute hydrochloric acid, water, alcohol, and ether in succession, until none of these solvents remove anything more. Fine paper or cotton wool yield very nearly pure cellulose by similar treatment.
Cellulose is a colourless, transparent mass, absolutely insoluble in water, alcohol, or ether. It is, however, soluble in a solution of cuprammoniac solution, prepared from basic carbonate or hydrate of copper and aqueous ammonia. The specific gravity of cellulose is 1.25 to 1.45. According to Schulze, its elementary composition is expressed by the percentage numbers:—
Carbon 44.0 per cent. 44.2 per cent.
Hydrogen 6.3 " 6.4 "
Oxygen 49.7 " 49.4 "
These numbers represent the composition of the ash free cellulose. Nearly all forms of cellulose, however, contain a small proportion of mineral matters, and the union of these with the organic portion of the fibre or tissue is of such a nature that the ash left on ignition preserves the form of the original. "It is only in the growing point of certain young shoots that the cellulose tissue is free from mineral constituents" (Hofmeister).
Cellulose is a very inert body. Cold concentrated sulphuric acid causes it to swell up, and finally dissolves it, forming a viscous solution. Hydrochloric acid has little or no action, but nitric acid has, and forms a series of bodies known as nitrates or nitro-celluloses. Cellulose has some of the properties of alcohols, among them the power of forming ethereal salts with acids. When cellulose in any form, such as cotton, is brought into contact with strong nitric acid at a low temperature, a nitrate or nitro product, containing nitryl, or the NO_{2} group, is produced. The more or less complete replacement of the hydroxylic hydrogen by NO_{2} groups depends partly on the concentration of the nitric acid used, partly on the duration of the action. If the most concentrated nitric and sulphuric acids are employed, and the action allowed to proceed for some considerable time, the highest nitrate, known as hexa-nitro- cellulose or gun-cotton, C_{12}H_{14}O_{4}(O.NO_{2})_{6}, will be formed; but with weaker acids, and a shorter exposure to their action, the tetra and penta and lower nitrates will be formed.[A]