Various forms of Pyroxyline.--Very soon after the first announcement of the discovery of Pyroxyline, most animated discussions arose amongst chemists with regard to its solubility and general properties. Some spoke of a "solution of Gun-Cotton in Ether;" whilst others denied its solubility in that menstruum; a third class, by following the process described, obtained a substance which was not explosive, and therefore could scarcely be termed Gun-Cotton. On further investigations some of these anomalies were cleared up, and it was found that there were varieties of Pyroxyline, depending mainly upon the degree of strength of the Nitro-Sulphuric Acid employed in the preparation. Still the subject was obscure until the publication of researches by Mr. E. A. Hadow. These investigations, conducted in the Laboratory of King's College, London, were published in the Journal of the Chemical Society. Constant reference will be made to them in the following remarks. We notice--first, the chemical constitution of Pyroxyline; secondly, its varieties; and thirdly, the means adopted to procure a Nitro-Sulphuric Acid of the proper strength. a. _Constitution of Pyroxyline._--Pyroxyline has been sometimes spoken of as a Salt of Nitric Acid, a Nitrate of Lignine. This view however is erroneous, since it can be shown that the substance present is not Nitric Acid, although analogous to it. It is the Peroxide of Nitrogen, which is intermediate in composition between Nitrous Acid (NO{3}) and Nitric Acid (NO{5}). Peroxide of Nitrogen (NO{4}) is a gaseous body of a dark red colour; it possesses no acid properties, and is incapable of forming a class of salts. In order to understand in what state this body is combined with cotton fibre to form Pyroxyline, it will be necessary to digress for a short time.