The formula for a definite Nitro-Sulphuric Acid, of the proper strength for making the soluble Pyroxyline, may be stated thus:-- HO NO{5}, 2 (HO SO{3}) + 3-1/2 HO or Atoms. Atomic weight. Nitric Acid 1 | 54 Sulphuric Acid 2 | 80 Water 6-1/2 | 58 | --- | 192 Having found the percentage of real acid which is present,[31] the following calculation will give the relative weights of the ingredients required to produce the formula:-- Let { _a_ = percentage of real Nitric Acid, { _b_ = " " Sulphuric Acid, then 5400/_a_ = quantity of Nitric Acid, 8000/_b_ = " Sulphuric Acid, 192 - 5400/_a_ - 8000/_b_ = " Water. [31] Tables are given in the Appendix for calculation by specific gravity; but direct analysis of the acids is the most certain. Observe that the numbers in the calculation correspond to the atomic weights recently given; and that the amount of water is derived from the _total atomic weight_, viz. 192, _minus_ the sum of the weights of both acids. Hence if the samples of acid employed are too weak for the purpose, the formula for the water gives a negative quantity. The weight of mixed acids produced by the formula is 192 grains, which would measure somewhere about two fluid drachms. Ten times this quantity forms a convenient bulk of liquid, in which about 50 or 60 grains of Paper may be immersed. In weighing corrosive liquids, such as Sulphuric and Nitric Acid, a small glass may be counterbalanced in the scale-pan, and the acid poured in carefully. If too much is added, the excess can be removed by a glass rod, or by "the pipette" commonly employed for such a purpose. The following example of a calculation similar to the above may be given:-- 100 parts of the Oil of Vitriol = 76·65 real acid. " " Nitric Acid = 65·4 real acid. therefore 8000/76·65 = 104·3 grains of Oil of Vitriol. 5400/65·4 = 82·5 " Nitric Acid 192 - 104·3 - 82·5 = 5·2 " Water. Multiplying these weights ten times, we have Oil of Vitriol 1043 grains. Nitric Acid 825 " Water 52 " ---- Total weight of the } 1920 grains. Nitro-Sulphuric Acid } Having prepared the acid mixture of a definite strength by the above formula, the paper must be immersed according to directions given at page 191. b. _Process for mixing Nitro-Sulphuric Acid, the strength of the two acids not having been previously determined._--Take a strong sample of Nitric Acid (the yellow Nitrous acid, so called, succeeds well), and mix it with Oil of Vitriol as follows:-- Sulphuric Acid 10 fluid drachms, Nitric Acid 10 "