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.
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