large excess of water, i.e., is at once immersed
entirely in the water, since at this stage it is especially liable to
decomposition, which, once started, is very difficult to stop. The warmer
the mixture and the less water it contains, the more liable it is to
decomposition; hence it is that on warm and damp days the centrifugal
machines are most likely to fire. The commencement of decomposition may be
at once detected by the evolution of red fumes. Directly the gun-cotton is
immersed in the large quantity of water in the beater and poacher it is
safe.

In order that the final product may be stable and have good keeping
qualities, it is necessary that it should be washed completely free from
acid. The treatment in the beater and poacher, by causing the material to
assume the state of a fine pulp, in contact with a large quantity of
water, does a good deal to get rid of the free acid, but the boiling
process is absolutely necessary. It has been proposed to neutralise the
free acid with a dilute solution of ammonia; and Dr C.O. Weber has
published some experiments bearing upon this treatment. He found that
after treatment with ammonia, pyroxyline assumed a slightly yellowish
tinge, which was a sure sign of alkalinity. It was then removed from the
water, and roughly dried between folds of filter paper, and afterwards
dried in an oven at 70 deg. C. After three hours, however, an explosion took
place, which entirely destroyed the strong copper oven in which the nitro-
cotton (about one oz.) had been drying. The explosion was in some respects
remarkable. The pyroxyline was the di-nitro-cellulose (or possibly the
penta-nitro?), and the temperature was below the igniting point of this
material (40 deg. C. would have been a better temperature). Dr Weber
determined the ignition point of his di-nitro-cellulose, and found it to
be 194 deg. to 198 deg. C., and he is therefore of opinion that the explosion was
due to the treatment of the partially washed material with ammonia. A
certain quantity of ammonium nitrate was probably formed, and subsequently
dried upon the nitro-cellulose, in a state of very fine subdivision. The
faintest trace of acid would then be sufficient to bring about the
explosive ignition of the ammonium nitrate.

The drying of gun-cotton or collodion-cotton is also a somewhat dangerous
operation. A temperature of 40 deg. C. (104 deg. F.) should not be exceeded, and
thermometers should be placed in the nitro-cot