532 Prof. J. J. Thomson on the Charge of Electricity 



and more sluggish than the original ones ; the presence of 

 these modified ions does not seem to give any appreciable 

 conductivity to the gas. Mr. Wilson found that when in gas 

 not exposed to Rontgen rays a dense cloud was produced in 

 dust-free air by a large expansion and then allowed to settle, 

 a subsequent small expansion (which under ordinary circum- 

 stances would not produce a cloud at all unless dust were 

 present) would produce a cloud, and that it was necessary to 

 produce several clouds and allow them to settle before the 

 gas returned to its normal state. In this case Mr. Wilson's 

 experiments seem to show that the original nuclei were 

 excessively minute drops of water, and the formation of the 

 subsequent cloud would seem to indicate that on those drops 

 which did not grow large enough to be carried down by the 

 lirst cloud some moisture was deposited, and that this was 

 prevented from evaporating by some kind of chemical change 

 at its surface such as the formation of hydrogen peroxide. 



Whatever the explanation of these secondary clouds may be 

 it is evident that when the rays are strong enough to produce 

 them we cannot deduce the number of ions from observations 

 on the primary cloud. In the experiments described below the 

 intensity of the rays was weakened by interposing screens of 

 aluminium between the bulb and the gas exposed to the rays 

 until there was no more cloud produced by the second 

 expansion than would have been produced if the gas had 

 never been exposed to the ravs. 



Another point which had to be investigated was whether the 

 expansion used was sufficient to bring down all the ions, or 

 whether the number brought down increased with the amount 

 of the expansion. To test this measurements were made of 

 the rate of fall of the clouds formed under exposure to the 

 rays by different expansions. The results of these experiments 

 are shown in the following table : — 



Pressure of air 768*08 millim. Temperature 18° C. 



tp ■ „ Time of fall through 25 millim. 



Expansion. with rays. without rays. 



752-72 _ . 



535-72 - 14 U W 



545^2-1.38 18 6 



The amount of water deposited per cub. centim. by an 

 expansion of 1*4 is 4*94 xlO -6 gram., while the amount 



