668 Mr. C. T. R. Wilson [March 7, 



WEEKLY EVENING MEETING, 



Friday, March 7, 1913. 



The Right Hon. Lord Rayleigh, O.M. D.C.L. LL.D. F.R.S., 



in the Chair. 



C. T. R. Wilson, Esq., M.A. F.R.S. 

 Photography of the Paths of Particles Ejected from Atoms. 



[Absteact.] 



The pheuomena of radioactivity are known to be due to the ejection 

 from the atoms of the radioactive elements of two kinds of particles 

 which travel with enormous velocities : (1) the alpha-particle, which 

 is a positively charged helium atom having a mass four times that of 

 the hydrogen atom ; (2) the beta-particle, which carries a negative 

 charge only half as large as the positive charge of the alpha-particle 

 and has a mass less than the 1700th part of the hydrogen atom. 



The velocity of the fastest beta-particles approaches very nearly 

 to that of Hght, that of the alpha-particles being considerably less but 

 still exceeding 10,000 miles a second. 



By the action of Rontgen and other radiations, we can cause 

 electrons or corpuscles which are identical with the beta-particles to be 

 expelled from the atoms of any element with velocities comparable 

 with those with which the alpha-particles are ejected from radium. 



The methods which have been used hitherto in the study of the 

 paths of these projectiles and of the effects produced by them in their 

 flight have been somewhat indirect. The actual paths of individual 

 particles have not been observed ; it has l)een necessary to investigate 

 the coml)iued effects of a large number of particles. 



It is true it has been found possible by two different methods to 

 detect effects arising from the action of a single alpha-particle. 

 Thus Rutherford introduced a method in which effects due to the 

 ions set free along the path of a single alpha-particle could be 

 detected by an electrometer : again in the Crookes spintharoscope 

 each alpha-particle causes a starlike point of light to flash forth 

 momentarily where it strikes the prepared screen. But it has not 

 been found possible by such methods to detect effects arising from a 

 single beta-particle. 



It is plain that a great advance would be made if it were possible 

 to induce each alpha- or beta-particle to leave a visible trail behind 

 it along its whole course and to photograph this trail. This is what 

 is accoinplislied by the method now described. 



