196 The Excitation of y Rays bg a Rays. 



When the ionization-chamber was filled with methyl iodide 

 the difference was a little greater, but the measurements 

 could not he made with the same accuracy owing to variations 

 in the pressure of the methyl iodide under the experimental 

 conditions. 



In order to prove that these 7 rays were excited by the 

 a rays and not by ft rays, the following experiments were 

 made: — 



The emanation tube was covered with sufficient aluminium 

 foil to stop the a rays. The measurements were then re- 

 peated, and no difference was observed when the positions 

 of the gold and aluminium tubes were changed. This showed 

 that the difference observed above could only be due to 

 a rays, or to very soft ft rays which were absorbed in the 

 aluminium foil used to stop the a rays. 



A simple experiment showed that the 7 rays w r ere not due 

 to soft ft rays. The emanation tube was placed in a con- 

 verging magnetic field so that the soft ft rays were coiled 

 by the field into a narrow spiral round the emanation tube. 

 In this way the slow ft rays were prevented from striking 

 the metal tubes. A difference was still observed between 

 the two ionizations, and was of the same magnitude. 



Tubes of silver, copper, and paper were also used. It was 

 found that the ionization produced by the emanation tube 

 was always greater when the material of the inner tube was 

 of higher atomic weight than the material of the outer tube. 



As an example, the measurements obtained when each of 

 the other metals was combined with aluminium may be given. 

 Taking the ionization observed when the inner tube was 

 aluminium as 100, the ionization obtained with gold as the 

 inner tube was 104*8, with silver 102'5, with copper 101*2, 

 and with paper 99*7. 



In order to obtain some idea of the penetrating power of 

 the excited 7 rays, a plate of aluminium 2 mm. thick was 

 placed over the face of the ionization-chamber. The dif- 

 ferences obtained by reversal of the tubes were now very 

 small, but the measurements indicated that the absorption 

 coefficient in aluminium of the excited rays was about 8 

 under the experimental conditions. 



The detailed investigation of the 7 rays excited by a rays 

 is, for obvious reasons, only possible with a source of a rays 

 which does not emit ft rays, such as ionium or polonium, 

 Experiments in conjunction with Mr. Russell have shown 

 that thorium oxide, containing a quantity of ionium equal to 

 the amount in equilibrium with 3*5 mgms. of radium, gives 

 7 rays greatly in excess of the ft rays. The amount of 7 rays 



