ment of a system of which the following are the chief points:
1. Isomeric substances have the samo molecular volumes.
2. Equal differences in composition correspond to equal differences in molecular volume.
The consequence of this is that the volume of a mole- cule is composed adddtively of the volumes of the atoms which it contains.
In order to determine atomic volumes H. Kopp ascer- tained that in homologous series the addition of the group CH 2 increases the molecular volume by 22 units.
„ _ Molecular
iLxample. — volume
He also found that the substitution of two atoms of hydrogen for one atom of carbon does not change the molecular volume.
Molecular Example* — volume
From these two experimentally ascertained facts, he was led to the conclusion :
CH 2 = 22, C = 11, H = 5-5.
By investigating such a substance as C 4 H 9 C1 and sub- tracting from its molecular volume the volume of four carbon atoms and nine hydrogen atoms, Kopp was able to find the atomic volume of chlorine.
By further application of this empirical principle, Kopp has determined the atomic volumes of a number of elements (0, CI, Br, I, S, N, &c), and has confirmed the general additive character of the molecular volume. He also recognised the necessity of taking into account certain constitutional influences. For instance, oxygen in the
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