Question of the Homogeneity of y-Rays. 749 



every body tried of density less than that of lead, the absorp- 

 tion coefficient per cm. of equivalent thickness for brass, for 

 instance, being at least 5 times less than that for lead. In 

 the experiments in which the electroscope was mounted on a 

 wooden stand this radiation is reflected from it through 

 the sides of the electroscope. In those conducted without 

 such a stand the radiation is reflected from the wood of the 

 table, and after reflexion it still retains its capacity for pene- 

 trating brass. The placing of much lead, brass, or copper on 

 the table cuts down the amount of this radiation, showing that 

 reflexion from wood is an essential part of the phenomenon. 

 It may be seen from Table IV. that this radiation may be 

 prevented from masking absorption results by cutting it off, 

 (1) at its point of production by surrounding the radium 

 entirely by a sufficient thickness of lead, (2) at its point of 

 reflexion by covering the table with lead, or (3) at the point 

 it enters the electroscope by using either a thick electroscope 

 of lead or by placing a 0*5 cm. circular screen of lead around 

 one made of some other metal having a thick lead base. The 

 thickness of lead required in (1) to prevent the escape of the 

 radiation was found to be about 05 cm. A thickness suffi- 

 cient to ensure the complete absorption of the /3-rays of 

 radium made no appreciable difference. The effect cannot 

 therefore be due to a secondary 7-radiation generated by the 

 action of /3-rays upon lead. It may be emphasized here that 

 this peculiar radiation is not caused by the action of an 

 untransformed primary radiation on a wood reflector, for if 

 it were, the same or a similar eff ect would be given when brass 

 absorbing plates were substituted for lead in I. (Table IV.). 

 The important point about this peculiar radiation is that it 

 appears to be produced only as an incidence radiation by 

 lead, and that it is very penetrating to brass, retaining this 

 power of penetration even after reflexion from wood. 



In addition to this radiation, secondary 7-ravs capable of 

 penetrating the brass electroscope are generated by the action 

 of primary emergent radiation from all bodies covering the 

 radium when it falls upon wood, glass, and magnesia brick. 

 Copper and brass produce such rays to a much less degree, 

 while lead produces no appreciable amount. These points 

 can be demonstrated by placing a thick plate of glass &c. 

 vertically on the table and measuring the leak with it present 

 and absent, taking care that none of the rays produced can 

 get through the windows rather than the walls of the electro- 

 scope. Like the peculiar radiation described, these rays also 

 are incapable of penetrating 0*5 cm. of lead. 



Tuomikoski, working with a very strong source of radium 

 emanation, has shown (Phys. Zeit. 1909, x. p. 372) that the 



