3 io SCIENCE PROGRESS 



duced by matter in all its states have been the subject of work 

 of a good many observers. Barkla's earlier work led him to 

 the result, since widely quoted, that 



(i) With elements of low atomic weight the secondary 

 radiation is wholly of the same type as the primary ; that, 

 in fact, the secondary radiation may be regarded as so much 

 scattered primary. 



(2) With elements of high atomic weight the secondary rays 

 are not all of the same type as the primary ; a portion of them 

 is very much softer. 



Barkla has since found {Nature, Feb. 13, 1908) that this 

 perfect scattering with light elements is only true when the 

 primary rays are very soft ; with very hard primary rays the 

 purely scattered radiation disappears, and is replaced by a 

 softer radiation, with a different distribution. Moreover, many 

 of the elements of high atomic weight will, under suitable 

 conditions, yield a scattered radiation, just as the lighter atoms 

 do. It thus appears probable that, with a suitable intensity of 

 the primary beams, all elements behave similarly. The soft 

 secondary radiation is in most cases characteristic of and 

 specially penetrating to its parent element. 



The secondary radiation from gases is of interest on account 

 of the comparative simplicity of the conditions. Barkla (1903-4) 

 concluded that the ratio of the quantity of the secondary 

 radiation to the quantity of the primary is independent of the 

 hardness of the primary, and dependent only on the density 

 of the gas producing it. The result has theoretical importance, 

 but Crowther (1907) has shown that the statement holds only 

 for those elements of low atomic weight (with the exception 

 of hydrogen), and that in general no " density law " can be said 

 to hold. As to the hardness of the secondary radiation, it varies 

 with the nature of the gas ; the lighter gases merely scatter 

 the primary radiation, while the heavier gases and vapours 

 transform it into very soft and intense secondary radiation. 



Heating effects of Rontgen rays. — The heat produced when 

 Rontgen rays are absorbed by metals was first measured by 

 Dorn (1897). The effects are small, and need a sensitive de- 

 tector. Rutherford and McClung (1902) used a bolometer, 

 Adams (1907) a radiomicrometer. Angerer (1906) has done 

 noteworthy and comprehensive work on the subject. Bumstead, 

 using a radiometer, obtained results in 1906 which seemed to 



