§ 74. The Stoichiometric Laws.
With the study of the relative weights in
which substances combine, certain generalisations or "natural laws" of
supreme importance were discovered. These stoichiometric laws, as they
are called, are as follows:—
- 1. "The Law of Constant Proportion"—The same chemical
compound always contains the same elements, and there is a constant
ratio between the weights of the constituent elements
present.
- 2. "The Law of Multiple Proportions"—If two substances combine
chemically in more than one proportion, the weights of the one which
combine with a given weight of the other, stand in a simple rational
ratio to one another.
- 3. "The Law of Combining Weights"—Substances combine either in
the ratio of their combining numbers, or in simple rational multiples or
submultiples of these numbers. (The weights of different substances
which combine with a given weight of some particular substance, which is
taken as the unit, are called the combining numbers of such substances
with reference to this unit. The usual unit now chosen is 8 grammes of
Oxygen.)2
As examples of these laws we may take the few following simple
facts:—
- 1. Pure water is found always to consist of oxygen and hydrogen
combined in the ratio of 1.008 parts by weight of the latter to 8 parts
by weight of the former; and pure sulphur-dioxide, to take another
example, is found always to consist of sulphur and oxygen combined in
the ratio of 8.02 parts by weight of sulphur to 8 parts by weight of
oxygen. (The Law of Constant Proportion.)
- 2. Another compound is known consisting only of oxygen and
hydrogen, which, however, differs entirely in its properties from water.
It is found always to consist of oxygen and hydrogen combined in the
ratio of 1.008 parts by weight of the latter to 16 parts by weight of
the former, i.e., in it a definite weight of hydrogen is combined
with an amount of oxygen exactly twice that which is combined
with the same weight of hydrogen in water. No definite compound has been
discovered with a constitution intermediate between these two. Other
compounds consisting only of sulphur and oxygen are also known. One of
these (viz., sulphur-trioxide, or sulphuric anhydride) is found always
to consist of sulphur and oxygen combined in the ratio of 5.35 parts by
weight of sulphur to 8 parts by weight of oxygen. We see, therefore,
that the weights of sulphur combined with a definite weight of oxygen in
the two compounds called respectively "sulphur-dioxide" and
"sulphur-trioxide," are in the proportion of 8.02 to 5.35, i.e.,
3:2. Similar simple ratios are obtained in the case of all the other
compounds. (The Law of Multiple Proportions.)
- 3. From the data given in (1) above we can fix the combining
number of hydrogen as 1.008, that of
sulphur as 8.02. Now, compounds are known containing sulphur and
hydrogen, and, in each case, the weight of sulphur combined with 1.008
grammes of hydrogen is found always to be either 8.02 grammes or some
multiple or submultiple of this quantity. Thus, in the simplest
compound of this sort, containing only hydrogen and sulphur (viz.,
sulphuretted-hydrogen or hydrogen sulphide), 1.008 grammes of hydrogen
is found always to be combined with 16.04 grammes of sulphur,
i.e., exactly twice the above quantity. (The Law of Combining
Weights.)
Berthollet (1748-1822) denied the truth of the law of constant
proportion, and a controversy ensued between this chemist and Proust
(1755-1826), who undertook a research to settle the question, the
results of which were in entire agreement with the law, and were
regarded as completely substantiating it.
