Table XIII contains one series of observations of this sort taken with my apparatus by Mr. P. I. Pierson. The first column gives the volts applied to the plates of the condenser shown in Fig. 7, p. 111. These were made variable so that the drop might always be pulled up with a slow speed even though its positive charge were continually increasing. The second and third columns give the times required to move 1 cm. under gravity and under the field respectively. The fourth column gives the time intervals required for the drop to experience a change in charge under the influence of a constant source of ultra-violet light—a quartz mercury lamp. The fifth column gives the total charge carried by the drop computed from equation (12), p. 91. The sixth column shows the change in charge computed from equation (10), p. 70. This is seen to be as nearly a constant as could be expected in view of Brownian movements and the inexact measurements of volts and times. The mean value of is seen to be , which yields with the aid of equation (16), p. 101, after the value of found for oil drops has been inserted, , which is in better agreement with the result obtained with oil drops than we had any right to expect. In these experiments the light was weak so that the changes come only after an average interval of 29 seconds and it will be seen that they are all unit changes. Table XIII MERCURY DROPLET OF RADIUS DISCHARGING ELECTRONS UNDER THE INFLUENCE OF ULTRA-VIOLET LIGHT Volts Drop No. 1 Sec. per Cm. Sec. per Cm. Time Interval between Discharges in Seconds Charge in No. Electrons Emitted 2.260 11.0 - 1200} {49.4} 3.070 11.0 + 32.8} {50.5} 11 4.4 1 1.960 + 194 54.4 12.8 1.960 + 190 60.8 6.4 1 23 1.820 11.2 + 220 65.0 4.2 1 40.0 1.690 + 230 69.8 4.8 1 15.2 1.550 + 332 75.1 5.3 1 3.040 Drop No. 2 10.4 + 98 43.5 5.6 2.540 + 200 49.4 5.9 1 18.6 2.230 + 300 55.2 5.8 1 35.0 2.230 + 76 60.7 5.5 1 42.0