This cloud-laden air then passes through a channel in an electrical insulator—a paraffin block —into the tubes , , , which contain concentrated sulphuric acid. These drying tubes remove all the moisture from the air and also such part of the charge as is held on ions which in the process of bubbling through , , have actually touched the sulphuric acid. The dry air containing the rest of the charge passes out through a channel in the paraffin block into the flask . (If the gas being studied was lighter than air, e.g., hydrogen, was of course inverted.) The outside of is covered with tin foil which is connected to one of the three mercury cups held by the paraffin block . If the air in contained at first no charge, then an electrical charge exactly [Pg 266] equal to the quantity of electricity which enters the flask will appear by induction on the tin-foil coating which covers this flask and this quantity can be measured by connecting the mercury cup 2 to cup 3 which is connected to the quadrant electrometer , and observing the deflection per minute. Precisely similarly the total quantity of electricity which is left per minute in the drying tubes , , is exactly equal to the quantity which appears by induction on the outer walls of the hollow metal vessel , which surrounds the tubes , , . This quantity can be measured by connecting mercury cup 1 to cup 3 and observing the deflection per minute of the quadrant electrometer. The number of cubic centimeters of gas which pass through the apparatus per minute is easily found from the number of amperes of current which are used in the electrolysis apparatus and the electro-chemical equivalent of the gas. By dividing the quantities of electricity appearing per minute in and by the number of cubic centimeters of gas generated per minute we obtain the total charge per cubic centimeter carried by the cloud. The increase in weight of the drying tubes , , per cubic centimeter of gas passing, minus the weight per cubic centimeter of saturated water vapor, gives the weight of the cloud per cubic centimeter. This completes the measurements involved in (2) and (3), p. 47. As to (4), p. 48, the average size of the droplets of water Townsend found by passing the cloud emerging from into a flask and observing how long it took for the top of the cloud to settle a measured number of centimeters. The radius of the drops could then be [Pg 267] obtained from a purely theoretical investigation made by Sir George Stokes,[198] according to which the velocity of fall of a spherical droplet through a gas whose coefficient of viscosity was is given by in which is the density of the droplet. From this Townsend got the average radius of the droplets and computed their average weight by the familiar formula . He was then ready to proceed as in (5), see p. 48. [Pg 268] APPENDIX C THE BROWNIAN-MOVEMENT EQUATION