326 Prof. E. Rutherford and Mr. H. Robinson on the 



necessity, as in the case of the emanation, to take into account 

 the decrease of the heat emission of the source. 



It is now necessary to consider what fraction of the energy 

 of the ft rays was absorbed in the arrangement of the heating 

 •apparatus shown in fig. 1, by which the heating effect of the 

 emanation was accurately determined. 



Eve* has made an estimate of the relative energy emitted 

 by the a, ft, and 7 rays from one gram of radium on the 

 assumption that the energy of the radiation emitted is pro- 

 portional to the total ionization produced. By this method 

 he deduced that the ft rays from radium contributed about 

 2 per cent., and the 7 rays about 4*5 per cent, of the total 

 heat emission. In his arrangement, however, the radium 

 was enclosed in a glass tube which must have absorbed a 

 considerable fraction of the soft ft rays^ and the correction 

 for the ionization of these soft rays was uncertain. It was 

 thought desirable to repeat the experiments made by Eve, 

 using the radium emanation in place of radium, and com- 

 pressing the former in a very thin glass tube, which allowed 

 the a. rays to escape freely. The stopping power of the glass 

 tube corresponded to only 2 cm. of air. Mr. Moseley kindly 

 assisted in these experiments, a more detailed account of which 

 will be published later by Mr. Moseley and Mr. Robinson. 



The method originally used by Eve is very suitable for 

 the purpose for which it was designed, and was employed in 

 these experiments. The ionization produced by the ft and 

 7 rays from the emanation-tube in a thin-walled ionization- 

 chamber supported in the middle of a room was measured 

 for different distances of the tube extending up to 12 metres. 

 The ionization current in the chamber was directly measured 

 by an electrometer using a balance method. Taking the 

 ionization in air due to the a. rays from radium in equi- 

 librium as 100, the total ionization in air due to complete 

 absorption of the ft rays was about 3'8, and for the 7 ra}\s 

 about 5*2. The ionization due to the 7 rays is somewhat 

 greater than that found by Eve, but the ionization due to 

 the ft rays is nearly twice as large. In Eve's experiment 

 a large part of the soft ft rays was absorbed in the radium 

 tube. By placing over the emanation-tube the copper and 

 glass tubes &c. used in measuring the heating effect of the 

 -emanation, it was found that about 85 per cent, of the total 

 ionization due to the ft rays was cut out. This was an 

 unexpectedly large fraction, but was confirmed in several 

 experiments. 



Remembering that the a rays from the emanation and its 

 products provide about 80 per cent, of the heating effect due 

 * Eve, Phil. Mag-, xxii. p. 851 (1911). 



