. On weighing before
and after such treatment, the difference in weights would give us the
silk present. The residue boiled with caustic soda would lose all its
wool, which is soluble in hot strong caustic alkali. Again straining
off, we should get only the cotton or other vegetable fibre left, and
thus our problem would be solved. Of course there are certain additional
niceties and modifications still needed, and I must refer you for the
method in full to the _Journal of the Society of Chemical Industry_,
1882, page 64; also 1884, page 517. I will now conclude with some tests
with alkaline and acid reagents, taken in order, and first the acids.
These will also impress upon our minds the effects of acids and alkalis
on the different kinds of fibres.

I. In three flasks three similar portions of cotton lamp-wick, woollen
yarn, and silk are placed, after previously moistening them in water and
wringing them out. To each is now added similar quantities of
concentrated sulphuric acid. The cotton is quickly broken up and
dissolved, especially if assisted by gentle warming, and at last a
brown, probably a black-brown, solution is obtained. The woollen is a
little broken up, but not much to the naked eye, and the vitriol is not
coloured. The silk is at once dissolved, even in the cold acid. We now
add excess of water to the contents of each flask. A brownish, though
clear, solution is produced in the case of cotton; the woollen floats
not much injured in the acid, whilst a clear limpid solution is obtained
with the silk. On adding tannic acid solution to all three, only the
silk yields a precipitate, a rather curdy one consisting of fibroin.

II. Three specimens of cotton, wool, and silk, respectively, are touched
with nitric acid. Cotton is not coloured, but wool and silk are stained
yellow; they are practically dyed.

III. Three specimens, of cotton, wool, and silk, respectively, are
placed in three flasks, and caustic soda solution of specific gravity
1.05 (10 deg. Twaddell) is added. On boiling, the wool and silk dissolve,
whilst the cellulose fibre, cotton, remains undestroyed.

IV. If, instead of caustic soda as in III., a solution of oxide of
copper in ammonia be used, cotton and silk are dissolved, but wool
remains unchanged, _i.e._ undissolved. If sugar or gum solutions be
added to the solutions of cotton and silk, the cotton cellulose is
precipitated, whilst the silk is not, but remains in solution.

V. Another