19 = I-] WILSON— CONSTITUTION OF THE ATOM. 367 



quently much less fundamental than, for example, Lord Kelvin's 

 vortex ring theory. The negative electrons and the positive sphere 

 may or may not turn out to be modes of motion of the aether; at 

 present we cannot say. 



One of the first questions which naturally arises in connection 

 with this theory is. how many negative electrons are there in each 

 atom? This question has been answered approximately by examin- 

 ing the effect of matter on light and Rontgen rays. When electric 

 waves pass over electrons the electrons are acted on by the electric 

 forces in the waves and so emit radiation. This means that the 

 electrons scatter the incident radiation. The amount of radiation 

 scattered by one electron can be calculated on the electromagnetic 

 theory and hence from the amount observed to be scattered by a 

 known amount of matter the number of electrons in the matter can 

 be estimated, the number of atoms in a given amount of matter 

 can be exactly calculated because we know the charge carried by 

 one atom in electrolysis and the total charge carried by the matter. 

 Hence we can get an estimate of the number of electrons per atom. 

 The total energy scattered by a mass containing .V electrons is 



o__ „i 



— jM — E where c is the charge on one electron, ;/; its mass and E 

 3 ^« ' 

 the incident energy. This formula is due to .Sir J. J. Thomson. 



The most recent determination of the energ}- scattered when 

 Rontgen rays pass through matter is that by Crowther.- He finds 

 that the number of electrons per atom of aluminium is 85, which is 

 about three times the atomic weight. Previous experiments of a 

 similar character have given nearly the same result for other 

 elements. It seems very probable therefore that all atoms contain a 

 number of electrons proportional to their atomic weights and not 

 very much greater. 



The mass of a negative electron is only one seventeen-hundredth 

 part of that of an atom of hydrogen, so that the negative electrons 

 only account for about one six-hundredth of the mass of any atom. 

 The rest of the mass therefore must be the mass of the positive 

 sphere. According to this theory therefore the mass of matter is not 

 electromagnetic in its origin, for the electromagnetic mass of the 



- Proc. Roy. Soc, A, Vol. 85, p. 29. 191 1. 



