192 ELECTRIC THEORY OF MATTER [CH. xx. 



A paper by Prof. Jeans, now of Princeton, N.J., in 

 -continuation of the discussion raised by Lord Rayleigh, 

 is to be found in the Philosophical Magazine for 

 April, 1906 ; also another paper in November, 1901. 



2. Attempt to determine the number of effective 

 corpuscles in an Atom. 



It is premature to do more than briefly refer to the 

 remarkable attempt made by Professor J. J. Thomson, 

 in the Philosophical Magazine for June, 1906, to 

 bring forward three lines of argument which tend to 

 show, on experimental grounds, that the number of 

 electrons in an atom is comparable with its atomic 

 weight, reckoning hydrogen as unity. It seems an 

 improbable result ; but the only way to get round it 

 is either to question the validity of the experiments 

 and the theory applied to them, or else to realise that 

 what is being measured is, not the total number of 

 electrons, but the number of free or available or 

 peculiarly constituted electrons. Cf. p. 162. 



One of the arguments may be simplified as follows : 



If the atom is composed of positive and negative 

 electricities, these constituent charges will tend to be 

 separated, against their mutual attraction, when sub- 

 jected to an external electric field such for instance 

 as the field existing in a wave of light ; and since 

 a light- wave is large compared with an atom, there 

 will be time for a certain amount of this separation 

 to be effected by each pulse. Accordingly the wave 

 will be as it were " loaded " by the electric charges, 

 its velocity of propagation will be reduced, and 

 refraction will occur. 



But the amount of loading, that is, the amount of 

 effective shear or alternating separation of the two 



