molecules. This law is the foundation of modern chemistry. A
cubic centimeter of O has as many molecules as a cubic centimeter
of H, a liter of N the same number as a liter of steam, under
similar conditions. Compare the number of molecules in 5 l. of
N2O with that in 10 l. Cl. 7 cc. vapor of I to 6 cc. vapor of S.
The half-molecules of two gases have, of course, the same
relation to each other, and in elements the half-molecule is
usually the atom.

The molecular volumes--molecules and the surrounding space--of
all gases must therefore be equal, as must the half-volumes.
Notice that this law applies only to gases, not to liquids or
solids. Let us apply it to the experiment for the electrolysis of
water. In this we found twice as much H by volume as O.
Evidently, then, steam has twice as many molecules of H as of O,
and twice as many half-molecules, or atoms. If the molecule has
one atom of O, it must have two of H, and the formula will be
H2O.

Suppose we reverse the process and synthesize steam, which can be
done by passing an electric spark through a mixture of H and O in
a eudiometer over mercury; we should need to take twice as much H
as O. Now when 2 cc. of H combine thus with 1 cc. of O, only 2
cc.of steam are produced. Three volumes are condensed into two
volumes, and of course three molecular volumes into two, three
atomic volumes into two. This may be written as follows:--

H + H + O = H2O.

This is a condensation of one-third.