2 Bumstead — Heating Effects produced by Rontgen Bays. 



breaking up of the atom of the secondary radiator ; but it is only 

 a suggestion, for it by no means follows that the presence of 

 /S-ravs involves atomic disintegration. The modern theories of 

 electrical conduction imply the existence in conductors (and all 

 bodies are conductors to a greater or less extent) of large num- 

 bers of corpuscles not closely bound up in the atomic structure ; 

 and it is quite conceivable that some of these may constitute the 

 secondary /3-rays, the necessary energy having been, in some 

 way, imparted to them by the primary radiation. The most 

 direct way of descriminating between these two possibilities 

 is to investigate the energy relations when, for example, Rontgen 

 rays are absorbed by matter. If none of the atoms are broken 

 up, then the conservation of energy, in the ordinary sense, 

 willhokl ; if, on the other hand, some of the atoms are exploded 

 by the Rontgen rays, as dynamite is exploded by a shock, 

 then the total energy after the absorption of the rays may be con- 

 siderably greater than the energy of the rays themselves. 

 This excess of energy might be expected to manifest itself mainly 

 in the form of heat in the absorbing body ; for it is known that 

 a large fraction of the secondary Rontgen rays are very easily 

 absorbed indeed ;*and Sagnac found that tertiary rays were more 

 easily absorbed than secondary rays.f Thus only the secondary 

 rays which are produced very near the surface of the absorber 

 would carry their energy away with them ; those which are 

 set up thoughout the mass of the body would be absorbed 

 before reaching the surface, and eventually would warm the 

 absorber. 



Assuming for the moment that Rontgen rays are able to 

 cause atoms to break up, it is very improbable that the atoms 

 of different substances are equally susceptible to this effect ; and 

 we should expect to find an inequality in the amount of heat pro- 

 duced when Rontgen rays are equally absorbed in different 

 substances. If, on the contrary, there is no atomic disintegra- 

 tion, the quantities of heat should be equal. It was from this 

 point of view that the problem was proposed to me by Professor 

 J. J. Thomson, during my stay in Cambridge last year, and the 

 experiments which I am about to describe were carried out in 

 the Cavendish Laboratory under his direction, and owe much 

 to his advice and cooperation. 



In considering the various experimental means by which 

 this problem could be attacked, the radiometer seemed to prom- 

 ise certain advantages over other heat-measuring instruments. 

 For measuring ordinary radiation with this instrument (the 

 development of which is chiefly due to E. F. Nichols) the usual 

 method is to have opaque vanes with a transparent wall near 



* J. J. Thomson: The Conduction of Electricity through Gases, p. 263. 

 flbid. p. 273. 



