301 



b cm 



Fig. I. 



in the figure. The plate 

 at A scatters in all direc- 

 tions the radiation which 

 it receives from the ra- 

 dium at R. Nearly all 

 the returned particles 

 strike the hemispherical 

 wall BB' of the ionisa- 

 tion chamber, the larger 

 lieniisphere CC being 

 connected to the electro- 

 meter. The usual shield- 

 ino- devices are used, but 



not shown in the figure. This arrangement gathers in the 

 ofrects of all the secondary rays, so that a measurable effect 

 is obtained, even when only a small quantity of radium is 

 used. 



When the cup or dome BB is made of very thin Al 

 foil, stretched on a frame of a few fine wires, the effects ob- 

 tained by placing plates of different metals at A approxi- 

 mate to those given by McClelland — that is to say, the 

 curve which shows the relation between secondary ionisation 

 and atomic weight, is of the same general form as McClel- 

 land's, though somewhat flatter. Bu.t when the dome is thick- 

 ened by the addition of layers of tinfoil, there is a conside- 

 rable alteration. The addition of tinfoil of a tenth of a 

 millimetre in thickness is sufhciont to make the curve almost 

 linear, and the ionisation is then nearly proportional to the 

 atomic weight of the radiator. 



The figures in the following table show the results of 

 an experiment of this kind. They give the ionisation cur- 

 rents, on an arbitrary scale, for different radiators and dif- 

 ferent thicknesses of the dome. The smaller figures are only 

 approximate : — 



- Ag. 



1010 



829 



272 



129 



71 



42 



Pb. 



Thin Al leaf 1300 



Sn. foil -0065 mm 1124 



•043 414 



•080 210 



•116 125 



•153 76 



Assuming that the returned radiation 

 a seattering of the primary beam, there are two ways of in- 

 terpreting Uiese results. The ^ radiation of radium is hete- 

 rogeneous, and consists of rays of various velocities. Accord- 

 ing to H. W. Schmidt there"^ are a certain number of groups, 

 each homogeneous in itself (Ann. der Phys., Nov., 1906). It 



