t first
knowing the atomic weights of the constituent atoms, and how many atoms
of each element are present in the molecule. But certain facts have been
discovered which suggest a way in which this can be done.

~Avogadro's hypothesis.~ We have seen that the laws of Boyle, Charles, and
Gay-Lussac apply to all gases irrespective of their chemical character.
This would lead to the inference that the structure of gases must be
quite simple, and that it is much the same in all gases.

In 1811 Avogadro, an Italian physicist, suggested that if we assume all
gases under the same conditions of temperature and pressure to have the
same number of molecules in a given volume, we shall have a probable
explanation of the simplicity of the gas laws. It is difficult to prove
the truth of this hypothesis by a simple experiment, but there are so
many facts known which are in complete harmony with this suggestion that
there is little doubt that it expresses the truth. Avogadro's hypothesis
may be stated thus: _Equal volumes of all gases under the same
conditions of temperature and pressure contain the same number of
molecules._

~Avogadro's hypothesis and molecular weights.~ Assuming that Avogadro's
hypothesis is correct, we have a very simple means for deciding upon the
relative weights of molecules; for if equal volumes of two gases contain
the same number of molecules, the weights of the two volumes must be in
the same ratio as the weights of the individual molecules which they
contain. If we adopt some one gas as a standard, we can express the
weights of all other gases as compared with this one, and the same
figures will express the relative weights of the molecules of which the
gases are composed.

~Oxygen as the standard.~ It is important that the same standard should be
adopted for the determination of molecular weights as has been decided
upon for atomic weights and equivalents, so that the three values may be
in harmony with each other. Accordingly it is best to adopt oxygen as
the standard element with which to compare the molecular weights of
other gases, being careful to keep the oxygen atom equal to 16.

~The oxygen molecule contains two atoms.~ One point must not be
overlooked, however. We desire to have our unit, the oxygen _atom_,
equal to 16. The method of comparing the weights of gases just suggested
compares the molecules of the gases with the _molecule_ of oxygen. Is
the molecule and the atom of oxygen the same