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° 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 alkaline solvent for silk, which, however, leaves undissolved
cotton and wool, is prepared as follows: 16 grains of copper sulphate
("blue vitriol," "bluestone") are dissolved in 150 c.c. of water, and
then 16 grains of glycerin are added. To this mixture a solution of
caustic soda is added until the precipitate first formed is just
re-dissolved, so as not to leave an excess of caustic soda present.




LECTURE III

WATER: ITS CHEMISTRY AND PROPERTIES; IMPURITIES AND THEIR ACTION; TESTS
OF PURITY