[Illustration: FIGS. 28, 29.--PLUNGE TANK, IN PLAN AND SECTION.]

~The Acid Mixture.~--Various formulæ 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 formulæ 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° 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