PRESIDKNTIAL ADDRESS. — SECTION' I. I7 



US now consider briefly the evidence on which this theory rests. 

 The transport of electrified particles may be brought about by 

 two processes : (i) by the passage of a current through a solution ; 

 (2) by the discharge of electricity through a vacuum tube. 



In (i) it is not dilftcult to obtain a ratio between the amount of 

 charge carried by each ion and the mass of the ion itself. In fact, 

 the ratio of e, the charge, to m, the mass, is lo^ for the hydrogen 

 ion. This may be determined experimentally. 



In the case of the discharge in the vacuum tube, the ratio of 



- for the negatively-electrified corpuscle is about i"? x lo" 



m -^ ' 



that is, the mass must be about ^ -',,,, of the mass of the atom 

 produced during electrolysis, so that the mass of the corpuscle is 

 about j-y„„ of that of the hydrogen atom. Now, it may be 

 shown that the charge upon the atom is the same during electro- 

 lysis as that upon the corpuscle in the vacuum tube, but, as the 

 ratio in the latter case is so much larger, it follows that the mass 

 of the corpuscle is ,-'„,, that of the atom. By several distinctly 



different methods it is proved that the ratio of is of the value 



m 



stated. From the rate at which a cloud falls after being produced 



by the expansion of moist air (according to the celebrated method 



of C. T. R. Wilson) we can find the size of the drops of water forming 



the cloud, and if we can measure the total charge carried by a 



definite number of drops, we can determine the charge of each drop. 



The value of e is thus found to be 3*1 x io~^" electrostatic 



units, or io~-'^ electromagnetic units — that is, it is the same as 



that carried by a hydrogen atom during electrolysis. Since 



^ = I-/ X 10" and t' = 10^-" it follows that in, the mass of the 



corpuscle, is about 6 x 10 -^. It is only the trained scientific 

 mind which can appreciate the magnitude of these numbers. 



A peculiarity of the corpuscle lies in the fact that the charge 

 associated with it is always negative, while that associated with 

 an atom may be positive or negative. (It has been shown by 

 J. J. Thomson that the mass of the corpuscle may be regarded as 

 being due to the charge of negative electricity residing upon it.) 



c 

 The value of for the positive atom is, however, 10^ instead of 

 m 



17 X 10^ for the corpuscle. Ev^ery atom of matter has associated 



with it at least one corpuscle, though the number is not limited 



to one. If a particle is set in motion by any means, the energy 



necessary to set it moving with the velocity v is h m v'^ ; but if the 



particle is electrified, the energy required will be greater, because 



a magnetic field will be generated by the moving charge, and by 



a well-known principle the production of the field will tend to 



stop the motion of the body giving rise to it. If the particle is 



a sphere of radius r, mass m, and the charge upon it e, then if it 



is moved with a velocity v, the energy required will be made up 



of two portions, (i) the energy necessary to give it a velocity 



