RONTGEN RAYS 309 



early steepening of the logarithmic curve was obtained by 

 McClelland, when working- with the /3-rays of radium ; for the 

 same metal the amount of the steepening agrees with the writer's 

 value for X rays. 



If the incident primary beam be not very hard, the whole of 

 the rays are transmitted exponentially, and the third portion 

 of the curve is undetectable. As the incident primary beam 

 increases in hardness, this third portion, indicative of hetero- 

 geneity, increases in extent and amount, at the expense of the 

 intermediate homogeneous region. 



When screen and radiator are of different materials the 

 logarithmic curve of transmission has throughout a gradually 

 diminishing slope indicative of complete heterogeneity : no 

 region of exponential absorption can be traced. The result 

 is probably to be attributed to the large amount of scattering 

 (or deviation from their original direction) which the rays 

 undergo when they encounter a screen which presents very 

 different atomic groupings to those of their parent anticathode. 



Secondary radiation. — When Rontgen rays come into contact 

 with matter, two classes of secondary rays are generated — one 

 a corpuscular radiation, the other a Rontgen radiation akin in 

 nature to the primary. 



1. Secondary corpuscular radiation. — Innes (1907) has shown that 

 in the case of metals subjected to a beam of Rontgen rays the 

 emitted negative electrons have a range of velocities in centimetres 

 per second of about 6 x io 9 to 7*5 x io 9 for soft rays and 6 x io 9 

 to 8 x io 9 for hard rays. Their number depends on the intensity 

 of the rays, their velocity does not : the speed does, however, 

 appear to increase a little with the atomic weight of the metal. 

 It is interesting to note that the velocity is of the same order as 

 those of the cathode particles in an X-ray tube. 



The distribution of these attendant corpuscles is not uniform. 

 Cooksey {Nature, April 2, 1908) found that the quantity of 

 electrons proceeding from the near or incidence side of a metal 

 plate is about 50 to 90 per cent, of the number leaving the 

 V emergence " side. More cathode rays are emitted from both 

 surfaces of metals of high atomic weight than from those of low. 

 Prof. J. J. Thomson (1906) showed that the intensity of the 

 "incidence" secondary cathode radiation increases steadily with 

 the atomic weight of the radiating substance. 



2. Secondary Rontgen radiation. — These secondary rays pro- 



