Molecules are grouped in such a way that those belonging to the same group may be considered as having the same state of movement; then an examination is made of the number of molecules in each group, and what are the changes in this number from one moment to another.

It is thus often possible to determine the part which the different groups have in the total properties of the system and in the phenomena which may occur.
Such a method, analogous to the one employed by statisticians for following the social phenomena in a population, is all the more legitimate the greater the number of individuals counted in the averages; now, the number of molecules contained in a limited space-- for example, in a centimetre cube taken in normal conditions--is such that no population could ever attain so high a figure.

All considerations, those we have indicated as well as others which might be invoked (for example, the recent researches of M.Spring on the limit of visibility of fluorescence), give this result:--that there are, in this space, some twenty thousand millions of molecules.

Each of these must receive in the space of a millimetre about ten thousand shocks, and be ten thousand times thrust out of its course.

The free path of a molecule is then very small, but it can be singularly augmented by diminishing the number of them.