27 



gauze chambers which have been described in fig. 4 to a 

 second gasometer. A gauze was inserted, and enabled the 

 draught to be carefully adjusted during each observation. 

 Readings were taken with the lower gauze at potentials of 50 

 and 600 volts respectively. The depth of the chamber was 

 11 mm. The mean value of the ratio of the readings with 

 the fields of 555 volts per cm. and 45 volts per cm. was 

 found to be the same for the mixture of ethyl-chloride and 

 air as it was for air only, its value being 1'05. Nitrous 

 oxide was similarly tested, and gave a similar result. The 

 velocity of the gas through the chamber was, in these experi- 

 ments, about 5 cm. per second. The uranium was 5 cm. 

 below the bottom gauze, so that approximately two seconds 

 elapsed before the products of ionization from the centre of 

 the uranium cylinder reached the gauze. Change in velocity 

 of the draught was found to have no appreciable effect upon 

 the result. The experiment indicates that air, ethyl-chloride, 

 and nitrous oxide, although giving distinctly different ioniza- 

 tion curves when the ionization is measured in the presence 

 and during the influence of the ionizing agent, show no 

 difference in behaviour from each other when the ionization is 

 collected from them after their removal fo*r a period of two 

 seconds or more from the influence of the ionizing agent. 



§*• 



It was now deemed advisable to make use of the second 

 method of experiment, as mentioned in the introduction. 



The general arrangement of the experiment was such 

 as to allow the a radiation from radium to act for some little 

 time upon the gas to be experimented upon, this being en- 

 closed in a suitable ionization-chamber, and both electrodes 

 meanwhile being connected to earth. A lead screen was 

 quickly interposed between the radium and the chamber : the 

 electrometer electrode was simultaneously disconnected from 

 earth, and a strong field immediately applied between the 

 electrodes. The field was left on for a time, sufficient to en- 

 sure all the ions to reach the electrodes. It was then re- 

 moved, and the charge, which had been collected on the in- 

 sulated electrode, was measured by a quadrant electrometer. 

 The radium bromide was held in a small metal cup placed 

 at A (fig. 8) some little distance below the floor of the ioni- 

 zation-chamber B. This chamber consisted of a brass case 

 connected to earth : in its floor was cut a circular opening 

 of 1"2 cm. diameter. The hole was covered by a thin sheet 

 of mica, F, which rendered the case gas-tight, but was thin 

 enough to enable the a radiation to penetrate between the 



