CHAPTER VII. 



How THE ELECTRICAL CHARGE IN A CORPUSCLE is ESTI- 

 MATED: A NEW USE FOR CLOUDS. 



In order to find the electrical charge on the corpuscle, it 

 is best to consider, now, the influence of ions on the forma- 

 tion of clouds. This cloud formation is a very striking ef- 

 fect produced on moist air by the presence of ions, whether 

 positive or negative, whenever the air is expanded a quarter 

 of its original volume. The fact seems to be that each 

 corpuscle acts as a nucleus about which the moisture of 

 the air collects, so that it becomes the centre of a visible 

 drop of water, and, where before there existed a moist, 

 invisible gas, there now suddenly appears a beautiful cloud. 



Though corpuscles due to any of the ionizing agencies 

 mentioned in Chapter I cause this cloud-condensation, one 

 of the best for the purpose is the X-rays, which are pecu- 

 liarly efficient in forming clouds in any gas through which 

 they pass. The mechanism of the experiment, in its sim- 

 plicity, is a mass of moist gas enclosed in a glass vessel, 

 through which are passing weak X-rays and in which the 

 moist air may suddenly be expanded with the formation of 

 a cloud of water drops. But the determination of the many 

 factors which it is necessary to know in' order to arrive at 

 the electrical charge on a corpuscle by this means, is most 

 formidable. 



The procedure is somewhat as follows : The velocity with 

 which each drop of water falls is measured; from this, the 

 radius of the drop may be calculated; knowing the radius 



(63) 



