6 THE PRESIDENTIAL ADDRESS; 
estimates of the size and mass of the atom gave little hope that we 
could detect the effect 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 « particle is so great that it can readily be detected by a variety 
of methods. Hach« particle, as Sir Wm. Crookes first showed, pro- 
duces a flash of light easily visible in a dark room when it falls on a 
screen coated with crystals of zinc sulphide. This scintillation method 
of counting individual particles has proved invaluable in many re- 
searches, for it gives us a method of unequalled delicacy for studying 
the effects of single atoms. The « particle can also be detected electri- 
cally 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 « particle but of any type of pene- 
trating radiation which produces ions or of electrified 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 photo- 
graphically. 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 confirmatory 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 projec- 
tiles 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 correct- 
ness of the atomic idea of matter. The method found capable of most 
accuracy depends on the definite proof of the atomic nature of elec- 
tricity 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 of the 
charges carried by electrons, « particles, and the ions produced in gases 
by X-rays and the rays from radioactive matter. It was first shown 
hy Townsend that the positive or negative charge carried by an ion in 
