10 OUTLINES OF PHYSICAL CHEMISTRY
of hydrogen and one volume of chlorine combine to form two volumes of hydrochloric acid gas. Two volumes of hydrogen and one volume of oxygen give two volumes of steam. Three volumes of hydrogen unite with one volume of nitrogen to give two volumes of ammonia.
Since experiment shows that there is a simple relation between the volumes of the gases which enter into reaction, and since it is, a priori, extremely probable that there is a simple relation between the numbers of atoms which combine with each other, 1 it follows that there ought to be a simple relation between the numbers of atoms which are contained in equal volumes of different gases.
One step further leads us to admit Avogadro's hypothesis : Equal volumes of different gases (measured at the same temperature and pressure) contain the same number of individual particles. If this hypothesis is correct, then :
1. The densities of different gases must stand in the same relationship as do the weights of their gaseous particles.
2. The individual particles of elementary gases must not be confounded with their atoms. They are more often composed of several atoms, and their degree of divisibility can be experimentally determined.
For instance : One volume of hydrogen unites with one volume of chlorine to form two volumes of hydrochloric acid. If Avogadro's hypothesis be well founded, this means that the chlorine originally concentrated in n particles has now become distributed over 2n particles. Consequently the individual particle of chlorine is divi-
1 In the case of chlorine and of hydrogen, for example, it is, a priori, very probable that the formation of a particle of hydrochloric acid gas results from the union of a small number of atoms of chlorine with a small number of atoms of hydrogen (1 to 1, 2 to 1, or some other simple ratio). The constancy of the composition of hydrochloric acid makes the contrary hypothesis (according to which,
for example, 976 .... atoms of chlorine to 899 atoms of hydrogen)
absolutely improbable.
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