Having now found means for proving the presence of the various elements composing cotton, silk, and wool, fur or hair, we come to methods that have been proposed for distinguishing these fibres more generally, and for quantitatively determining them in mixtures.  One of the best of the reagents for this purpose is the basic zinc chloride already referred to.  This is made as follows:  100 parts of fused zinc chloride, 85 parts of water, and 4 parts of zinc oxide are boiled together until a clear solution is obtained.  This solution dissolves silk slowly in the cold, quickly if hot, and forms a thick gummy liquid.  Wool, fur, and vegetable fibres are not affected by it.  Hence if we had a mixture, and treated with this solution, we could strain off the liquid containing the dissolved silk, and would get cotton and wool left.  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.