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

I have already had occasion to refer, in my last Lecture, to water as a chemical substance, as a compound containing and consisting of hydrogen and oxygen.  What are these water constituents, hydrogen and oxygen?  Each of them is a gas, but each a gas having totally different properties.  On decomposing water and collecting the one of these two gases, the hydrogen gas, in one vessel, and the other, the oxygen gas, in another vessel, twice as large a volume of hydrogen gas is given off by the decomposing water as of oxygen.  You may now notice a certain meaning in the formula assigned to water, H_{2}O:  two volumes of hydrogen combined with one of oxygen; and it may be added that when such combination takes place, not three volumes of resulting water vapour (steam), but two volumes are produced.  This combination of the two gases, when mixed together, is determined by heating to a high temperature, or by passing an electric spark; it then takes place with the consequent sudden condensation of three volumes of mixture to two of compound, so as to cause an explosion.  I may also mention that as regards the weights of these bodies, oxygen and hydrogen, the first is sixteen times as heavy as the second; and since we adopt hydrogen as the unit, we may consider H to stand for hydrogen, and also to signify 1—­the unit; whilst O means oxygen, and also 16.  Hence the compound atom or molecule of water, H_{2}O, weighs 18.  I must now show you that these two gases are possessed of totally different properties.  Some gases will extinguish a flame; some will cause the flame to burn brilliantly, but will not burn themselves; and some will take fire and burn themselves, though extinguishing the flame which has ignited them.  We say the first are non-combustible, and will not support combustion; the second are supporters of combustion, the third are combustible gases.  Of course these are, as the lawyers say, only ex parte statements of the truth; still they are usually accepted.  Oxygen gas will ignite a red-hot match, but hydrogen will