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,