[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. CHEMISTRY OF THE SOLUTION OF PYROXYLINE IN ETHER AND ALCOHOL, OR "COLLODION." The substitution compounds B and C, already alluded to as forming the Soluble Cotton of Photographers, are both abundantly soluble in Acetic Ether. This liquid however is not adapted for the purpose required, inasmuch as on evaporation it leaves the Pyroxyline in the form of a white powder, and not as a transparent layer. The rectified Ether of commerce has been found to answer better than any other liquid as a solvent for Pyroxyline. If the sp. gravity be about ·750, it contains invariably a small proportion of _Alcohol_, which appears to be necessary; the solution not taking place with absolutely pure Ether. The Pyroxyline, if properly prepared, begins almost immediately to gelatinize by the action of the Ether, and is soon completely dissolved. In this state it forms a slimy solution, which, when poured out on a glass plate, dries up into a horny transparent layer. In preparing Collodion for Photographic purposes, we find that its physical properties are liable to considerable variation. Sometimes it appears very thin and fluid, flowing on the glass almost like water, whilst at others it is thick and glutinous. The causes of these differences will now engage our attention. They may be divided into two classes: first, those relating to the Pyroxyline; second, to the solvents employed.