2. He determined with the aid of a quadrant electrometer the total electrical charge per cubic centimeter carried by the gas. 3. He found the total weight of the cloud by passing it through drying tubes and determining the increase in weight of these tubes. [Pg 48] 4. He found the average weight of the water droplets constituting the cloud by observing their rate of fall under gravity and computing their mean radius with the aid of a purely theoretical law known as Stokes’s Law. 5. He divided the weight of the cloud by the average weight of the droplets of water to obtain the number of droplets which, if assumption 1 is correct was the number of ions, and he then divided the total charge per cubic centimeter in the gas by the number of ions to find the average charge carried by each ion, that is, to find . A brief description of the way in which these experiments were carried out is contained in Appendix B. One of the interesting side results of this work was the observation that clouds from negative oxygen fall faster than those from positive oxygen, thus indicating that the negative ions in oxygen act more readily than do the positive ions as nuclei for the condensation of water vapor. This observation was made at about the same time in another way by C. T. R. Wilson,[32] also in the Cavendish Laboratory, and it has played a rather important rôle in subsequent work. Wilson’s discovery was that when air saturated with water vapor is ionized by X-rays from radioactive substances and then cooled by a sudden expansion, a smaller expansion is required to make a cloud form about the negative than about the positive ions. Thus when the expansion increased the volume in a ratio between 1.25 and 1.3, only negative ions acted as nuclei for cloudy condensation, while with expansions greater than 1.3 both negatives and positives were brought down. [Pg 49] Townsend first obtained by the foregoing method, when he worked with positive oxygen, and when he worked with negative oxygen, In later experiments[33] he obtained 2.4 and 2.9, respectively, in place of the numbers given above, but in view of the unavoidable errors, he concluded that the two charges might be considered equal and approximately . Thus he arrived at about the same value for as that which was then current because of the kinetic theory estimates of , the number of molecules in a cubic centimeter of a gas.