166 ANNUAL REPORT SMITHSONIAN INSTITUTION, 192-1 



The influence of radioactivity has also extended to yet another field 

 of study of fascinating interest. We have seen that the first rough 

 estimates of the size and mass of the atom gave little hope that we 

 could detect the efi'ect of a single atom. The discovery that the radio- 

 active bodies expel actual charged atoms of helium with enormous 

 energy altered this aspect of the problem. The energy associated 

 with a single a particle is so great that it can readily be detected by 

 a variety of methods. Each a particle, as Sir William Crookes first 

 showed, produces a flash of light easily visible in a dark room when 

 it falls on a screen coated with crystals of zinc sulphide. This scin- 

 tillation method of counting individual particles has proved invalu- 

 able in many researches, for it gives us a method of unequaled deli- 

 cacy for studying the effects of single atoms. The a particle can also 

 be detected electrically or photographically, but the most powerful 

 and beautiful of all methods is that perfected by Mr. C. T. R. Wilson 

 for observing the track through a gas not only of an a particle but 

 of any type of penetrating radiation which produces ions or of elec- 

 trified particles along its path. The method is comparatively simple, 

 depending on the fact, first discovered by him, that if a gas saturated 

 with moisture is suddenly cooled each of the ions produced by the 

 radiation becomes the nucleus of a visible drop of water. The water- 

 drops along the track of the a particle are clearly visible to the eye, 

 and can be recorded photographically. These beautiful photographs 

 of the effect produced by single atoms or single electrons appeal, I 

 think, greatly to all scientific men. They not only afford convincing 

 evidence of the discrete nature of these particles, but give us new 

 courage and confidence that the scientific methods of experiment 

 and deduction are to be relied upon in this field of inquiry; for 

 many of the essential points brought out so clearly and concretely in 

 these photographs were correctly deduced long before such confirma- 

 tory photographs were available. At the same time, a minute study 

 of the detail disclosed in these photographs gives us most valuable 

 information and new clues on many recondite effects produced by the 

 passage through matter of these flying projectiles and penetrating 

 radiations. 



In the meantime a number of new methods had been devised to fix 

 with some accuracy the mass of the individual atom and the number 

 in any given quantity of matter. The concordant results obtained by 

 widely different physical principles gave great confidence in the 

 correctness of the atomic idea of matter. The method found capable 

 of most accuracy depends on the definite proof of the atomic nature 

 of electricity and the exact valuation of this fundamental unit of 

 charge. We have seen that it was early surmised that electricity was 

 atomic in nature. This view was confirmed and extended by a study 



