Thereafter we have only to count these drops to ascertain the number of ions which existed in the gaseous mass.
To effect this counting, several methods have been used, differing in principle but leading to similar results.

It is possible, as Mr C.T.R.
Wilson and Professor J.J.Thomson have done, to estimate, on the one hand, the weight of the mist which is produced in determined conditions, and on the other, the average weight of the drops, according to the formula formerly given by Sir G.Stokes, by deducting their diameter from the speed with which this mist falls; or we can, with Professor Lemme, determine the average radius of the drops by an optical process, viz.

by measuring the diameter of the first diffraction ring produced when looking through the mist at a point of light.
We thus get to a very high number.

There are, for instance, some twenty million ions per centimetre cube when the rays have produced their maximum effect, but high as this figure is, it is still very small compared with the total number of molecules.

All conclusions drawn from kinetic theory lead us to think that in the same space there must exist, by the side of a molecule divided into two ions, a thousand millions remaining in a neutral state and intact.
Mr C.T.R.Wilson has remarked that the positive and negative ions do not produce condensation with the same facility.