RONTGEN RAYS 311 



show that more heat was generated when the rays were totally 

 absorbed by lead than by zinc ; it was suggested that the results 

 were due to the liberation of atomic energy, and thus lent 

 support to the explosive theory (p. 316). However, Bumstead's 

 later work (1908) and that of Angerer (1907) have not confirmed 

 the results, and the assumption that the heat developed is 

 proportional to the energy in the rays absorbed may be regarded 

 as legitimate. 



The available energy seems in all the different reactions — 

 thermal, photographic, ionisation, etc. — to be proportionally 

 larger with soft than with hard rays. 



Polarisation of Rontgen rays. — Barkla in 1905 showed that 

 the scattered secondary Rontgen rays from a plate bombarded 

 by primary rays exhibit an asymmetry of distribution in a plane 

 at right angles to the direction of propagation of the primary 

 beam. This asymmetry is such as to make the intensity a 

 minimum in a direction parallel to the cathode rays in the 

 X-ray tube, and a maximum in a direction at right angles to 

 this. For a carbon radiator, for example, these intensities have, 

 with soft primary rays, the relative amounts 1 : 1*2. The values 

 approach equality, or in other words the extent of the polari- 

 sation diminishes as the hardness of the primary ray increases. 

 While we are concerned with the distribution of these "inci- 

 dence" scattered secondary rays it is worth noting that their 

 intensity along the path of the primary rays is for soft rays 

 about double that in a plane at right angles : with hard rays 

 the ratio diminishes a little. 



If a secondary beam is allowed to fall on a reflector, tertiary 

 rays are generated : their distribution indicates an almost 

 complete polarisation of the secondary rays. Barkla measured 

 his intensities by an ionisation method. Haga (1907) has re- 

 peated the experiments using a photographic method. He has 

 obtained striking confirmation of Barkla's results for the polari- 

 sation of secondary beams, but his experiments do not indicate 

 any polarisation in the case of the primary rays. 



This polarisation or one-sidedness of a Rontgen ray has 

 theoretical importance, and will be brought forward for dis- 

 cussion when the rival theories are considered. 



Velocity of Rontgen rays. — In 1906 Marx published the results 

 of an ingenious and difficult investigation on the speed of the 

 Rontgen rays. He excited the rays by means of waves from an 



