Compounds B and C, either separate or in a state of mixture, form the soluble material employed by the Photographer. They both dissolve in Acetic Ether, and also in a mixture of Ether and Alcohol. The latter, viz. C, also dissolves in glacial Acetic Acid. They are produced by a Nitro-Sulphuric Acid slightly weaker than that used for A, and contain a smaller amount of Peroxide of Nitrogen.
Compound D resembles what has been termed Xyloidine, that is, the substance produced by acting with Nitric Acid upon Starch. It contains less Peroxide of Nitrogen than the others, and dissolves in Ether and Alcohol, and also in Acetic Acid. The ethereal solution leaves, on evaporation, an opaque film, which is highly combustible, but not explosive.
By bearing in mind the properties of these compounds, many of the anomalies complained of in the manufacture of Gun-Cotton disappear. If the Nitro-Sulphuric Acid employed is too strong, the product will be insoluble in Ether; whilst if it is too weak, the fibres are gelatinized by the Acid and partly dissolved.
c. Means adopted to procure a Nitro-Sulphuric Acid of the requisite strength for preparing Pyroxyline.—This is a point of more difficulty than would at first appear. It is easy to determine an exact formula for the mixture, but not so easy to hit upon the proper proportions of the acids required to produce that formula; and a very slight departure from them altogether modifies the result. The main difficulty lies in the uncertain strength of commercial Nitric Acid. Oil of Vitriol is more to be depended upon, and has a tolerably uniform Sp. Gr. of 1·836;[13] but Nitric Acid is constantly liable to variation; hence it becomes necessary to make a preliminary determination of its real strength, which is done either by taking the specific gravity and referring to tables, or, better still, by a direct analysis. As each atom of Sulphuric Acid removes only a given quantity of water, it follows that the weaker the Nitric Acid, the larger the amount of Sulphuric which will be required to bring it up to the proper degree of concentration.
[13] The later experience of the writer induces him to believe, that the specific gravity of Oil of Vitriol cannot always be taken as an indication of its real strength; which is best ascertained by analysis.
To avoid the trouble necessarily attendant upon these preliminary operations, many prefer to use, in place of Nitric Acid itself, one of the salts formed by the combination of Nitric Acid with an alkaline base. The composition of these salts, provided they are pure and nicely crystallized, can be depended on.
Nitrate of Potash, or Saltpetre, contains a single atom of Nitric Acid united with one of Potash. It is an anhydrous salt, that is, it has no water of crystallization. When strong Sulphuric Acid is poured upon Nitrate of Potash in a state of fine powder, in virtue of its superior chemical affinities it appropriates to itself the Alkali and liberates the Nitric Acid. If care be taken to add a sufficient excess of the Sulphuric Acid, a solution is obtained containing Sulphate of Potash dissolved in Sulphuric Acid, and free Nitric Acid. The presence of the Sulphate of Potash (or, more strictly speaking, of the Bi-Sulphate) does not in any way interfere with the result, and the effect is the same as if the mixed acids themselves had been used.
The reaction may be thus represented:—
| Nitrate of Potash | plus | Sulphuric Acid in excess | |
| = | Bisulphate Potash | plus | Nitro-Sulphuric Acid. |