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.
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