By a reference to the formula, it is seen that the fifth atom of Oxygen contained in the Nitric Acid takes one of Hydrogen, and forms an atom of Water; the NO{4} then steps in, to fill the gap which the atom of Hydrogen has left. All this is done with so little disturbance that even the fibrous structure of the cotton remains as before. b. _Chemical Composition of the varieties of Pyroxyline._--Mr. Hadow has succeeded in establishing _four_ different substitution compounds, which, as no distinctive nomenclature has been at present proposed, may be termed compounds A, B, C, and D. _Compound A_ is the most explosive Gun-Cotton, and contains the largest amount of Peroxide of Nitrogen. It dissolves _only in Acetic Ether_, and is left on evaporation as a white powder. It is produced by the strongest Nitro-Sulphuric Acid which can be made. _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.