The formulæ for Negative and Positive Collodion, and for the Nitrate Bath and developing fluids, are given in the second Chapter. THE SOLUBLE PAPER. Pyroxyline may be prepared either from cotton wool or from Swedish Filtering-paper. Most operators prefer the latter, from its giving a product of constant solubility, and yielding a fluid solution.[30] The Cotton Wool however is better adapted for use with the Sulphuric Acid and Nitre, since the Paper, from its closeness of texture, requires a longer immersion in the mixture. [30] Swedish filtering-paper may be procured at the operative chemists', at about five shillings the quire. Each half-sheet has the water-mark "J. H. Munktell." Preparation of a Nitro-Sulphuric Acid of the proper strength.--There are two modes of preparing the Nitro-Sulphuric Acid: first, by mixing the acids; second, by the Oil of Vitriol and Nitre Process. The former is the best in cases where large quantities of the material are operated on, but the amateur is recommended to begin by trying the Nitre Process (p. 190) as the most simple. PREPARATION OF NITRO-SULPHURIC ACID BY THE MIXED ACIDS. The operator may proceed in either of two ways; first, by taking the strength of each sample of acid, and mixing according to fixed rule; second, by a more ready plan, which may be used when the exact strength of the acids is not known. Each of these will be described in succession. a. _Directions for mixing according to fixed rule._--This process is given from Mr. Hadow's original paper in the 'Quarterly Journal of the Chemical Society.' It is certain in its results if the strength of both acids be accurately determined. A very perfect process for taking the strength of Nitric Acid is by means of powdered Marble or Carbonate of Lime, as described in various works on practical Chemistry. Sulphuric Acid may be estimated by precipitating with Nitrate of Baryta, and weighing the insoluble Sulphate with the proper precautions. The specific gravity is not a criterion of strength to be perfectly relied on, but if it be adopted as a test, the following points must be attended to. 1st. That the temperature of the acid be at or near 60° Fahrenheit; the density of Sulphuric Acid especially is, from its small specific heat, greatly influenced a change of temperature. 2nd. The sample of Nitric Acid must be free from Peroxide of Nitrogen, or only slightly coloured by it. This substance, when present, increases the specific gravity of the acid without adding to its available properties. A yellow sample of Nitric Acid will therefore be somewhat weaker than is indicated by the specific gravity. 3rd. The Oil of Vitriol should yield no solid residue on evaporation. Sulphate of Lead and Bisulphate of Potash are often found in the commercial acid, and add much to its density. Oil of Vitriol containing Sulphate of Lead becomes milky on dilution.