Third. A definite relative weight, called the atomic
and molecular weight of the element and molecular weight
of the compound : as, CI for 35.5 parts by weight for the
atom of chlorine ; 71 parts by weight for the molecule of
chlorine; and 58.5 parts by weight for the molecule of
sodium chloride.
Fourth. In the case of those elements or compounds
which can exist in the gaseous state, a certain volume, as
will be shown in the following : —

From the law (Avogadro's Law) that equal volumes of
all gaseous substances^ at same temperature and pressure^
contain the same number of molecules^ it follows that the
molecules of all gaseous substances occupy the same
amount of space. A litre of hydrogen and a litre of
chlorine have the same number of molecules. The hy-
drogen molecule and the chlorine molecule occupy the



2 OHBMICAIi PROBLEMS.
same amount of space, although not necessarily of the
same size (since the distance between two hydrogen mole-
cules may not be the same as the distance between two
chlorine molecules). K we assume that the hydrogen
molecule occupies two volumes, then the molecule of
chlorine must occupy two volumes, and so must the
molecule of every other gaseous substance. In the case
of hydrogen, since the molecule has two atoms. Ha stands
for a molecule and for two volumes, while H stands for
one volume. The same is true for all other elements
having two atoms to the molecule ; as, hydrogen, chlorine,
bromine, iodine, oxygen, sulphur,^ selenium, tellurium,
nitrogen, arsenic, phosphorus, and antimony .2

When there is only one atom in the molecule, the sym-
bol stands for two volumes. As in mercury, Hg stands for
two volumes, so also with zinc and cadmium.
