Polonium, Radium^ and Radioactinium. 119 



subjected to different processes of purification. This /ir- 

 radiation was soft, but its amount was too small to be inves- 

 tigated further. It was, however, much more penetrating 

 than the /3-rays described by Bumstead and McGougan*, 

 which were shown to be completely absorbed by 

 0*64 X 10~ 4 cm. aluminium. 



Quantitative determination of the Intensities of 

 strong sources of Polonium. 



The polonium was next transferred from aluminium to 

 different metals to see whether or not the a-raysof polonium 

 set up 7-radiation in these metals. It was necessary, there- 

 fore, to have a means of measuring accurately by a-rays the 

 strength of the sources employed. The quantities of polonium 

 to be measured varied Avidely in intensity, from about that 

 in equilibrium with *01 mg. radium to that in equilibrium 

 with 2 mg. The usual methods of measuring bodies which 

 expel a-rays, e. g. the scintillation method of counting par- 

 ticles, are much too sensitive for quantities of this magnitude. 

 The ionization current produced by such large amounts is, 

 however, sufficiently great to be measured by a galvanometer, 

 and this was the method employed. To enable quantities of 

 widely different magnitudes to be measured without change 

 in the apparatus, a balance method was used, which consisted 

 simply in sending a current through a galvanometer in the 

 opposite direction to the ionization current. The current 

 was provided by a storage-cell and could be varied by means 

 of a resistance-box. We found in this way that the actual 

 amount of polonium separated by us from 18 mgrms. of 

 radium metal after six years' growth was that in equilibrium 

 with 1*3 mgrms. of radium. The theoretical amount, pro- 

 vided the polonium had been separated quantitatively, is 

 3'6 mgrms., taking the period of radium D as 16'5 years. 

 There is a serious discrepancy between these numbers. We 

 found a discrepancy of a similar order between the theore- 

 tical and experimental amounts of polonium separated from 

 5 mgrms. of old radium which had been left untouched for 

 seven years. We cannot offer any adequate explanation of 

 these results. It is most unlikely that Ave obtained a third 

 only of the total quantity of polonium, since the separations 

 were repeated without obtaining a further quantity of 

 polonium. On the other hand, AntonofPs method of deter- 

 mining the period of radium I) was equivalent to the above, 

 and his value used in our calculation is very similar to that 

 obtained previously by Meyer and von Schweidler. 

 * Bumstead & McGougan, Phil. Mag. vol. xxiv. p. 402. 



