718 Professor C. G. Barkia [Mav 26, 



])articles have been found to possess the mass of the atom of 

 liydrogen : positive particles are not set free either by light or by 

 X-rays ; even when liberated in radioactive processes they are atoms 

 of helium. 



Positive electricity is thus much more securely fixed in matter. 

 For these and other reasons, Kelvin suggested, and J. J. Thomson 

 developed, the idea that an atom was built up of a uniform sphere of 

 positive electricity with a large number of negative electrons. As the 

 more diffusely distributed a charge is the less is its mass, such a 

 cliarge would passess little mass— approximately the whole mass of 

 the atom would reside in the electrons. It was therefore necessary 

 to assume a large number of these, say 1700 per atom of hydrogen, 

 and they Avere supposed to be arranged in rings or shells revolving 

 round the centre of the atom. 



It is on this point as to the number of electrons per atom that 

 X-radiation phenomena afford most valuable evidence. 



When X-rays are incident on matter, the matter itself emits 

 X-radiations (secondary X-radiations). I have found the secondary 

 X-radiations are made up of radiations of two types, which I have 

 called (a) scattered X-radiation, and (J?) fluorescent (characteristic) 

 X-radiation. 



The principal secondary radiation, proceeding from light elements 

 exposed to a primary X-radiation, is a radiation of the same wave- 

 length and the same penetrating power. From observation of the 

 character and intensity of this scattered radiation, I concluded that 

 per electrons in matter were the scatterers, and that the scattering 

 the atom (consequently the number of electrons per atom), for light 

 atoms at least, was proportional to the atomic weight. 



J. J. Thomson stated this more precisely by pointing out that 

 each charged particle, when under the influence of the primary 

 X-radiation, is acted upon by the varying electric field, is moved by 

 that field, and so sends out a wave-motion along its own tubes of 

 force. This wave-motion is the scattered radiation. 



From measurements of the intensity of radiation scattered appHed 

 to J. J. Thomson's calculated value for the energy of a beam lost by 



scattering from ions (the fraction lost per cm. of path -= -N -^/x^) 



I found, with data available, there were about 200 electrons per 

 molecule of air. Applying more recent and exact data, we find 

 approximately 14 -.5 x (2-75 x lO^-') electrons per c.c. of air under 

 standard conditions. 



This and the corresponding measurements for scattering bv other 

 substances indicates 7 electrons per atom of N, 8 for 0, 6 for C, 

 16 for S, 1 for H. It is remarkable that the determined number of 

 electrons is as near to 7 for an atom of nitrogen as it is possible 

 to estimate the pressure of the air upon which the experiments 

 w^ere made. 



