Page:Outlines of Physical Chemistry - 1899.djvu/31

 sible into two parts, that is, it contains at least two atoms.

The same reasoning applies to the hydrogen.

The reaction 2 volumes of hydrogen + 1 volume of oxygen = 2 volumes of water- vapour, shows in the same way that the gaseous particle of oxygen is divisible into two parts, i.e. contains two atoms. The study of ammonia leads us to the same conclusion for nitrogen. Hence we write H 2, 01 2 , 2 , N 2 , to denote the single individual particles of these gases. These single individual particles correspond to what we now actually term gaseous molecules.

So marjy excellent arguments have been brought forward in favour of Avogadro's hypothesis that it becomes almost as true as a well-ascertained fact.

All gases behave in the same way when compressed or expanded, a fact which indicates a great similarity, or even an identity, of physical structure, and finds its simplest explanation in the admission that in equal volumes of all gases there is the same number of molecules.

Besides, if chemical phenomena had not called for Avogadro's hypothesis, the kinetic theory of gases would inevitably have led to it (Clausius, Maxwell).

As a third proof it may be added, that all chemical theories which have been based on this hypothesis have introduced simplicity and harmony into the science. It will be profitable now to give an outline of these theories.

Determination op Molecular Weights

The molecular weight of a gas is proportional to its density.

The lightest substance known, hydrogen, is chosen as standard of comparison. As we denote the atomic weight of hydrogen by 1, the molecule of this gas, containing two atoms, will weigh 2.

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