us a method by which we can determine the relative
weights of the molecules of two gases because it enables us to tell when
we are dealing with an equal number of the two kinds of molecules. If by
any other means we can get this information, we can make use of the
knowledge so gained to determine the molecular weights of the two
substances.

~Raoult's laws.~ Two laws have been discovered which give us just such
information. They are known as Raoult's laws, and can be stated as
follows:

1. _When weights of substances which are proportional to their molecular
weights are dissolved in the same weight of solvent, the rise of the
boiling point is the same in each case._

2. _When weights of substances which are proportional to their molecular
weights are dissolved in the same weight of solvent, the lowering of the
freezing point is the same in each case._

By taking advantage of these laws it is possible to determine when two
solutions contain the same number of molecules of two dissolved
substances, and consequently the relative molecular weights of the two
substances.

~Law of Dulong and Petit.~ In 1819 Dulong and Petit discovered a very
interesting relation between the atomic weight of an element and its
specific heat, which holds true for elements in the solid state. If
equal weights of two solids, say, lead and silver, are heated through
the same range of temperature, as from 10 deg. to 20 deg., it is found that very
different amounts of heat are required. The amount of heat required to
change the temperature of a solid or a liquid by a definite amount
compared with the amount required to change the temperature of an equal
weight of water by the same amount is called its specific heat. Dulong
and Petit discovered the following law: _The specific heat of an element
in the solid form multiplied by its atomic weight is approximately equal
to the constant 6.25._ That is,

at. wt. x sp. ht. = 6.25.

Consequently,

6.25
at. wt. = --------
sp. ht.

This law is not very accurate, but it is often possible by means of it
to decide upon what multiple of the equivalent is the real atomic
weight. Thus the specific heat of iron is found by experiment to be
0.112, and its equivalent is 27.95. 6.25 / 0.112 = 55.8. We see,
therefore, that the atomic weight is twice the equivalent, or 55.9.

~How formulas are determined.~ It will be well in connection with
molecular weights to con