he wool not having been acted
on by the acid remains quite intact. The process is known as
"carbonising". It may not only be done by means of the acids named but
also by the use of acid salts, such as aluminium chloride, which on
being heated are decomposed into free acid and basic oxide. For the same
reason it is important to avoid the use of these bodies, aluminium
chloride and sulphate, zinc and magnesium chlorides, etc., in the
treatment of cotton fabrics; as in finishing processes, where the goods
are dried afterwards, there is a great liability to form hydrocellulose
with the accompaniment of the tendering of the goods.

ACTION OF NITRIC ACID.

The action of nitric acid on cellulose is a variable one, depending on
many factors, strength of acid, duration of action and temperature.
Naturally as nitric acid is a strong oxidising agent the action of
nitric acid on cellulose is essentially in all cases that of an oxidant,
but the character of the product which is obtained varies very much
according to the conditions just noted. When cellulose or cotton in any
form is immersed in nitric acid of 1.4 to 1.5 specific gravity for a
moment, and the fibre be well washed, there is a formation of hydrate of
cellulose which has a gelatinous nature. This is deposited on the rest
of the material, which is not materially affected so far as regards
strength and appearance, but its power of affinity for dyes is
materially increased. There is some shrinkage in the size of the cotton
or paper acted upon.

Nitric acid changes all kinds of cellulose into nitro products, the
composition of which depends upon the strength of the acid, the duration
of treatment, and one or two other factors. The nitrocelluloses are all
highly inflammable bodies, the more highly nitrated burning with
explosive force. They are produced commercially and are known as "gun
cotton" or "pyroxyline". The most highly nitrated body forms the basis
of the explosive variety; the least highly nitrated forms that of the
soluble gun cotton used for making collodion for photographic and other
purposes.

The products formed by the action of nitric acid are usually considered
to be nitrocelluloses. It would appear that they are more correctly
described as cellulose-nitrates, for analysis indicates the presence of
the NO{3} group, which is characteristic of nitrates, and not of the
NO{2} group, which is the feature of nitro bodies in general. Further,
nitro compounds,