SOME CONTEMPORARY ADl'.-tXCFS IX rilVSICS I'lll 41.1 

 C. Positively-Charged Particles Accepted as Atoms' 



Posit ivcly-charRcd particles are found in abundance in gases in 

 which an electrical discharRe is or has lately been maintained, and 

 they nuiy Ik- prtxiiiced under weil-controlletl circumstances by pouring 

 a stream of electrons with properly-adjusted s|ieeds into a gas, and 

 in other ways. Only the ratio of the ciiarge to the mass can be 

 iK'termined for these particles, not the charge individually nor the 

 mass individually. But particles of apparently the same substance 

 show distinct values of this ratio, which stand to one another as 

 the numbers 1, 2, 3, . . . and the intermediate values do not occur. 

 This sup[K)rts the quite natural idea that these particles are atoms 

 which have lost one or two or three or more of their electrons. If 

 we make this sup()osition, we thereb\' assume values for the charges, 

 and can calcidate the masses of the particles from these and the 

 observed values of the charge-mass ratio. The masses lie between 

 10 -'and 10~-' (in grammes) for particles occurring in the vapors of 

 the various chemical elements, and they lie in the same order as the 

 coinbining-weights of the chemical elements. This is powerful testi- 

 mony that the particles indeed deser\e the name of "atoms". 



There is one sort of positive particle for which the charge can be 

 measured directly. This is the alpha-particle, which cannot be pro- 

 duced at will but is supplied by Nature from rtidio-active substances. 

 Counting the number of these particles emitted from a bit of radio- 

 active substance in a given time, and measuring the total electrical 

 charge lost by the substance in the same time, and dividing the latter 

 figure by the former, Rutherford and Regener obtained the charge 

 of the alpha-particle, which is twice the electron-charge (with reversed 

 sign) within the limits of experimental error. This suggests that the 

 alpha-particle is an atom of something or other, which has lost two 

 electrons. As an evacuated tube into which alpha-particles are ad- 

 mitted is presently found to contain helium, the "something or other" 

 is supposed to be helium. The mass of the alpha-particle can be de- 

 termined directly from its charge and charge-mass ratio. It amounts 

 to G.OOlO -', and this agrees with the mass inferred in the foregoing 

 way for the positive particles found in helium. 



4.774-10"'". .Anyone interested in his case may find it presented in the .'\pril, 1925, 

 numlier of the Philosophical Magazine. The question is for e.xperimental physicists 

 to discuss; but it is not likely that the edifice of mo<lern physics is liable to Ix; ruined 

 by a flaw at its very' foundation, such as this would l)C. 



' The material of this section may l>c found much more extensively presented in 

 my fourth article, in which I have also written about isotopes, a subject omitted 

 here for the sake of brevity. 



