[eve] primary and secondary gamma rays 49 



From No. 14 we see that the reversals of plates of nearly equal 

 atomic weights, such as Fe, Cu, produce very slight changes within 

 the electroscope. 



The hardening effect of lead is well seen by contrasting Nos. 1 

 and 2, for on increasing the thickness from 0.6 to 2.6 cm., II. changes 

 from 98. 5 to 106.5 and III. from 83.5 to 78.0. No such alterations 

 take place when the thickness of iron is increased as in Nos. 3 and 

 4. The same general results are found in Nos. 5 to 9 when carbon 

 and lead are used. 



Hence we may conclude that the ;- rays themselves which pass 

 through the electroscope are unaffected by the reversal of plates 

 A and B ; also that each face contributes its own corpuscular 

 secondary radiation to an extent depending only on the atomic 

 weight of the substance of which the interior surface layer of that 

 face is composed. This secondary radiation is almost independent 

 of the thickness, provided the latter exceeds a millimetre or two. 

 Moreover, the secondary radiations are so absorbed that there is 

 little or no evidence of any continuous rebound from face to face. 

 Each surface produces its own secondary radiation to an extent 

 which is mainly independent of the character of the other faces. 



Summary. 



1. Change of hardness of the j rays makes little difference in 

 the relative intensities of the incident corpuscular secondary radia- 

 tion from various elements, and the intensities follow the order of 

 atomic weights of the radiators. (Table I. and curve I., fig. 2.) 



2. Change in hardness of the y rays makes a marked difference 

 in the relative intensities of the emergent corpuscular secondary 

 radiation from various elements. Such radiations do not follow 

 the order of atomic weight. (Table III., fig. 3.) 



3. Hardening of the y rays is readily produced by lead screens, 

 but not by iron screens of great thickness. 



4. On hardening the ;' rays the emergent radiation from carbon 

 and aluminium is increased relatively to heavier substances; on 

 softening, decreased. (Figs. 3, 5). 



5. With various radiators, and with the secondary rays due 

 to the primary ;- rays from radium C, the values of X and Xjà, and 

 the emergent corpuscular secondary radiations were observed, 

 (fig. 5). These all indicate the soft character of the secondary ;- rays. 



6. The y rays from Uranium X caused emergent radiation of a 

 character showing that these rays are softer than those from radium 

 C, and harder than the secondary y. 



Sec, III., 1909. 4. 



