1911.] Anniversary Address by Sir A. Geikie. 



447 



respectively as centres of condensation ; and he showed that equal numbers 

 of ions were produced by X-rays and by the rays from radioactive substances. 

 The effect of other agencies in producing nuclei in gases was examined in 

 detail. The results of these experiments, which are now classical, were 

 communicated in a series of memoirs published in the ' Philosophical 

 Transactions.' 



This condensation property of ions, discovered by Wilson, was utilised by 

 Sir J. J. Thomson to count the number of ions present, and to determine 

 that fundamental electrical unit, the charge carried by an ion in gases. 

 Eecently Mr. Wilson has perfected the expansion method to detect the 

 effects of individual «- and /3-particles. The path of each «- or /3-particle 

 through the gas is marked out by condensation of water upon the ions 

 it produces, and the trails showing the paths of the particles can be 

 photographed. He has also obtained photographs illustrating the distribu- 

 tion of ions due to the passage of X-rays through a gas, which show clearly 

 the trails of the /3-particles liberated from the atoms of matter. These 

 experiments are of the greatest interest and importance, and visualise in a 

 remarkable way the fundamental properties of these radiations. 



A further study by this extraordinarily delicate method promises not only 

 to afford a practical means of counting the «- and /3-particles in a gas, but 

 also to throw light upon some of the more important and recondite effects 

 produced by the passage of different types of ionising radiation. 



Mr. Wilson was one of the first to investigate the so-called natural 

 ionisation of gases ; he devised a simple type of electroscope for this 

 purpose, which has come into general use, and he has constructed a tilted 

 electroscope of great sensibility, which is now widely used for measurements 

 of ionisation. He has also directed his attention to atmospheric electricity ; 

 he has devised an instrument for measuring accurately the current which 

 passes from the upper atmosphere to the earth, and has determined the 

 value of this current under different conditions. 



