carried by the Ions produced by Rontgen Rays. 531 



connected from the battery expansion produced a dense cloud; 

 when, however, the plates were connected with the battery 

 only a very light cloud was produced by the expansion, and 

 this cloud was almost as dense when the Rontgen rays did not 

 pass through the air as when they did. 



Another point which had to be investigated was whether 

 the cloud produced by the expansion caught all the ions. In 

 this connexion it is necessary to point out that it is only 

 possible to use expansions comprised within somewhat 

 narrow limits. The ratio of the final to the initial volume of 

 the gas has to be between 1*25 and 1*40. For, as Mr. Wilson 

 (loc. cit.) has shown, when the expansion exceeds the larger 

 of these values a dense cloud is produced even when the gas 

 is not exposed to Rontgen rays, with these large expansions 

 the cloud is so dense that the increase produced by the 

 Rontgen rays is barely perceptible ; while when the expansion 

 is less than the smaller of these values no cloud at all is pro- 

 duced. With expansions comprised between these limits it 

 was found that when the Rontgen rays were strong an increase 

 in the strength of the rays did not increase the number of 

 drops in the cloud, as determined by the rate of fall of the 

 drops, nearly so fast as it increased the number of ions as 

 measured by the electrical conductivity of the gas. But with 

 these strong rays it was found that the effect of the Rontgen 

 rays in producing a cloud was not exhausted by the first 

 expansion, even when the rays were cut off immediately after 

 that expansion took place ; for a cloud was produced when a 

 second expansion was made, and with strong rays it some- 

 times required six or seven expansions, occupying perhaps 

 five or six minutes, before the effect of the rays had dis- 

 appeared. In the face of this it is evident that when the rays 

 are strong we are not entitled to assume that all the ions are 

 brought down by the cloud produced by the first expansion. 

 The first expansion, however, though it does not bring all the 

 ions down, seems to increase the size of those left and makes 

 them more permanent, for the ions which are left after the 

 first expansion exert an appreciable cloud-producing effect 

 for several minutes ; whereas if no expansion had occurred 

 the effect of the rays in producing a cloud would only have 

 lasted for a few seconds after the rays had been cut off. 

 Again, these modified ions are able to cause a cloud to settle 

 with an expansion less than 1*25, the minimum expansion 

 which gives a cloud with the original ions. When once a 

 cloud has been produced the secondary clouds produced by 

 subsequent expansions are but little affected by an electric 

 field, this again indicating that the modified ions are larger 



