C.If the laws of solution were identically the same for a solution of sea-salt, the same depression should be noticed in a saline solution also containing 1 molecule per litre.

In fact, the fall reaches 3.26 deg., and the solution behaves as if it contained, not 1, but 1.75 normal molecules per litre.

The consideration of the osmotic pressures would lead to similar observations, but we know that the experiment would be more difficult and less precise.
We may wonder whether anything really analogous to this can be met with in the case of a gas, and we are thus led to consider the phenomena of dissociation.[18] If we heat a body which, in a gaseous state, is capable of dissociation--hydriodic acid, for example--at a given temperature, an equilibrium is established between three gaseous bodies, the acid, the iodine, and the hydrogen.

The total mass will follow with fair closeness Mariotte's law, but the characteristic constant will no longer be the same as in the case of a non-dissociated gas.

We here no longer have to do with a single molecule, since each molecule is in part dissociated.
[Footnote 18: Dissociation must be distinguished from decomposition, which is what occurs when the whole of a particle (compound, molecule, atom, etc.) breaks up into its component parts.