CHAPTER XXIII. LAWS OF DEFINITE AND OF MULTIPLE PROPORTION.

113. Weight and Volume.--We have seen that water contains two
parts of H by volume to one part of O; or, by weight, two parts
of H to sixteen of O. These proportions are invariable, or no
symbol for water would be possible. Every compound in the same
way has an unvarying proportion of elements.

114. Law of Definite Proportion.--In a given compound the
proportion of any element by weight, or, if a gas, by volume is
always constant. Apply the law, by weight and by volume, to
these: HCl, NH3, H2S, N2O.

There is another law of equal importance in chemistry, which the
compounds of N and O well illustrate.


Weight. Volume.
N. O. N. O.
Nitrogen protoxide N2O 28 16 2 1
Nitrogen dioxide N2O2 28 32 2 2
Nitrogen trioxide. N2O3 28 48 2 3
Nitrogen tetroxide N2O4 28 64 2 4
Nitrogen pentoxide N2O5 28 80 2 5


Note that the proportion of O by weight is in each case a
multiple of the first, 16. Also that the proportion by volume of
O is a multiple of that in the first compound. In this example
the N remains the same. If that had varied in the different
compounds, it would also havevaried by a multiple of the smallest
proportion. This is true in all compounds.

115. Law of Multiple Proportion.--Whenever one element combines
with another in more than one proportion, it always combines in
some multiple, one or more, of its least combining weight, or, if
a gas, of its least combining volume.

The least combining weight of an element is its atomic weight;
and it is this fact of a least combining weight that leads us to
believe the atom to be indivisible.

Apply the law in the case of P2O, P2O3, P2O5; in HClO, HClO2,
HClO3, HClO4, arranging the symbols, weights, and volumes in a
table, as above.

The volumetric proportions of each element in the oxides of
nitrogen are exhibited below.


_ + _ + _ = __
N + N + O = N2O

_ + _ + _ + _ = __
N + N + O + O = N2O2

_ + _ + _ + _ + _ = __
N + N + O + O + O = N2O3

_ + _ + _ + _ + _ + _ = __
N + N + O + O + O + O = N2O4

_ + _ + _ + _ + _ + _ + _ = __
N + N + O + O + O + O + O = N2O5