ton. To some extent the dyeing
of cotton with the basic dyes of the second group and the mordant dyes
of the third group is almost a mechanical one, the cotton fibre taking
no part in it from a chemical point of view, but simply playing the part
of a base or foundation on which the colour lake may be formed. In the
case of the dyes of the fourth group, there being no chemical affinity
of the cotton known for them, these dyes cannot be used in a successful
manner; cotton will, if immersed in a bath containing them, more or less
mechanically take up some of the colour from the liquor, but such colour
can be almost completely washed out again, hence these dyes are not used
in cotton dyeing, although many attempts have been made to render them
available.

Indigo is a dye-stuff that stands by itself. Its combination with the
cotton fibre is chiefly of a physical rather than a chemical nature; it
does not form colour lakes in the same way as Alizarine and Magenta do.

Cellulose can be dissolved by certain metallic solutions and
preparations:--

(1) =Zinc Chloride.=--When cotton or other form of cellulose is heated
with a strong solution, 40 to 50 per cent., it slowly dissolves to a
syrupy liquid. On diluting this liquid with water the cellulose is
thrown down in a gelatinous form, but more or less hydrated, and
containing some zinc oxide, 18 to 25 per cent., in combination.

(2) =Zinc Chloride and Hydrochloric Acid.=--When zinc chloride is
dissolved in hydrochloric acid a liquid is obtained which dissolves
cellulose; on dilution the cellulose is re-precipitated in a hydrated
form. It is worth noting that the solution is not a stable one: on
keeping, the cellulose changes its character and undergoes hydrolysis to
a greater or less extent.

(3) =Ammoniacal Copper.=--When ammonia is added to a solution of copper
sulphate, there is formed at first a pale blue precipitate of copper
hydroxide, which on adding excess of ammonia dissolves to a deep blue
solution--a reaction highly characteristic of copper. The ammoniacal
copper solution thus prepared has, as was first observed by John Mercer,
the property of dissolving cellulose fairly rapidly, even in the cold.

If instead of preparing the ammoniacal copper solution in the manner
indicated above, which results in its containing a neutral ammonium
salt, the copper hydroxide be prepared separately and then dissolved in
ammonia a solution is obtained which is stronger in its acti