[Illustration: FIGS. 28, 29.--PLUNGE TANK, IN PLAN AND SECTION.] ~The Acid Mixture.~--Various formulae have been published for producing soluble nitro-cellulose. In many instances, although the observations were correct for the single experiment, a dozen experiments would have produced a dozen different products. The composition of the acids used depends upon the substance to be nitrated, and the temperature at which the nitration will be worked. Practically there are three formulae in general use--the one used by the celluloid manufacturers; another in which the cotton is nitrated at high temperatures; and a third in which the temperature of the immersion is low, and the time of nitration about six hours. Of the three, the best method is the last one, or the one in which the cotton is immersed at a low temperature, and then the reaction allowed to proceed in pots holding from 5 to 10 lbs. of cotton. The formula used by the celluloid manufacturers for the production of the low form of nitrated product which they use is:-- Sulphuric acid 66 parts by weight. Nitric acid 17 " " Water 17 " " Temperature of immersion, 30 deg. C. Time, twenty to thirty minutes. The cellulose is used in the form of tissue paper 2/1000 inch thick, 1 lb. to 100 of acid mixture. The nitro-cellulose produced by this formula is very insoluble in the compound ethers and other solvents of pyroxyline, and is seemingly only converted or gelatinised by the action of the solvent. The next formula produces a mixture of tetra-and penta-nitro- celluloses hardly soluble in methyl-alcohol (free from acetone), but very soluble in anhydrous compound ethers, ketones, and aldehydes:-- Nitric acid, sp. gr. 1.435 8 lbs. Sulphuric acid, sp. gr. 1.83 15-3/4 lbs. Cotton 14 oz. Temperature of nitration, 60 deg. C. Time of immersion, forty-five minutes. The 60 deg. of temperature is developed by mixing the acids together. The cotton is allowed to remain in the acid until it feels "short" to the rod. The following table, due to Mr W.D. Field, shows very plainly the great variation in the time of the immersion and the temperature by seemingly very slight causes. It extends over fourteen working days, during which time it rained four days. The formula used is that given above, except that the specific gravity of the nitric acid is somewhat lower. The product obtained differs only from that produced by using nitric acid of specific gravity 1.43 in being soluble in methyl-alcohol. From 30 to 35 lbs. of pyroxyline were produced in each of the fourteen days.