PRESIDENTIAL ADDRESS. — SECTION I. IQ 



The theory has been developed from several points of view : 

 firstly, from secondary Rontgen radiation ; secondly, by deter- 

 mining the opacity of substances to cathode rays ; thirdly, from 

 general optical properties. 



By considering the results obtained by studying secondary 

 Rontgen radiation, the following conclusions were arrived at. 

 Rontgen rays, passing through a gas, give rise to secondary rays 

 owing to the pulses set up by the rays acting upon the charges 

 possessed by the corpuscles causing the latter to move with great 

 velocity, and these in turn will give off a similar kind of radiation 

 to the original Rontgen radiation. It can be shown that the 



r ii T ■ • 1 8 T Xt'* . , 



energy of the new radiation is equal to „ times the energy 



passing through unit volume in unit time. The nature of the 

 secondary radiation will, of course, vary with the nature of the 

 material, but it has been shown that for elements of low atomic 

 weight the quality — i.e., penetrating power of the secondary 

 radiation — is the same as that of the primary. It has also been 

 shown that the proportion between the energy of the secondary 

 radiation and the primary radiation is independent of the nature 

 of the primary radiation, and for different substances the ratio 

 between the primary and secondary is directly proportional to the 



8 77- Xe^ 

 density of the substance. As the ratio is equal to - ' it 



3 m 

 follows that the number of corpuscles in a unit volume is pro- 

 portional to the density of the substance, and density is equal to 

 the product of the number of atoms by the atomic weight. From 

 this it follov/s that the number of corpuscles is proportional to the 

 atomic weight. B3/ studying the refractive index of a gas, for light 

 waves of a frequency p, it has been shown that the probable number 

 of corpuscles in the case of the hydrogen atom is about i, and 

 similar reasoning applied to other elements indicate that the 

 number of corpuscles is the same as the atomic weight. 



It has been pointed out that the mass of a corpuscle is about 

 "iTTro of that of an atom of hydrogen, and therefore if there is one 

 corpuscle composed of negative electricity to each atom of hydrogen, 

 it follows that the greater part of the atom must be composed of 

 something else. Now the atom is a neutral body, electrically 

 speaking, so that the remainder }S;|;J of the whole atom must be 

 composed of -h electricity or a charge of -f- equal to that of the — 

 on the corpuscle must reside upon the atom. This, of course, is 

 on the assumption that there is only one corpuscle to each atom 

 — this has been shown to be probably not the case. Kaufmann 



measured — for corpuscles from radium at different speeds and 

 m 



showed that it apparently increased with the speed, and we gather 



from his experiments that at the velocity attained the electrical 



mass was one-fifth that of the mechanical mass. Its energy must 



also increase, therefore, with the speed. Now, if we can conceive 



of the speed becoming very great and if the apparent or electrical 



mass increases in proportion, a point will at last be reached when 



