But in order to form the cloud, Thomson utilized C. T. R. Wilson’s discovery just touched upon above, that a sudden expansion and consequent cooling of the air in (Fig. 1) would cause the ions in to act as nuclei for the formation of water droplets. To produce this expansion the piston is suddenly pulled down so as to increase the volume of the space above it. A cloud is thus formed about the ions in . Instead of measuring the weight of this cloud directly, as Townsend had done, Thomson computed it by a theoretical consideration of the amount of cooling produced by the expansion and [Pg 52] the known difference between the densities of saturated water vapor at the temperature of the room and the temperature resulting from the expansion. This method of obtaining the weight of the cloud was less direct and less reliable than that used by Townsend, but it was the only one available with Thomson’s method of obtaining an ionized gas and of measuring the charge per cubic centimeter on that gas. The average size of the droplets was obtained precisely as in Townsend’s work by applying Stokes’s Law to the observed rate of fall of the top of the cloud in chamber . [Pg 53] The careful consideration of Thomson’s experiment shows that it contains the theoretical uncertainties involved in Townsend’s work, while it adds some very considerable experimental uncertainties. The most serious of the theoretical uncertainties arise from (1) the assumption of Stokes’s Law, and (2) the assumption that the number of ions is equal to the number of droplets. Both observers sought for some experimental justification for the second and most serious of these assumptions, but subsequent work by H. A. Wilson, by Quincke, and by myself has shown that clouds formed by C. T. R. Wilson’s method consist in general of droplets some of which may carry one, some two, some ten, or almost any number of unit charges, and I have never been able, despite quite careful experimenting, to obtain conditions in which it was even approximately true that each droplet carried but a single unit charge. Quincke has recently published results from which he arrives at the same conclusion.[35]