Heating Effect of Radium and its Emanation. 327 



to the a rays from radium in equilibrium, it follows that the 

 heating effect of the /3 and 7 rays absorbed in the experi- 

 mental arrangement described in § 2 was 4*2 per cent, of 

 that due to the a rays. This is, of course, based on the 

 assumption that the total ionization produced by the a. and 

 /3 rays is a measure of their relative energy. 



From the ionization measurements, it follows that the total 

 heating effect of the 7 rays from the emanation should be 

 6*5 per cent, of that due to the a rays. It is difficult to 

 estimate with certainty the fraction of the 7 rays absorbed 

 by the thickness of mercury of 1*46 cm., but it was probably 

 about 70 per cent. The heating effect of the 7 rays should 

 thus be about 4*6 per cent, of that of the emanation. This 

 is in fair accord with the observed increase of heating: effect 

 of: 5 per cent., of which probably about *5 per cent, was due 

 to /3 rays. 



It should be pointed out that the increase of the heating 

 effect of 2 per cent, observed when the copper cylinder was 

 replaced by a lead cylinder 1*2 mm. thick, is greater than 

 would be expected from the ionization results. The value 

 was undoubtedly nearly correct, for it was verified in a 

 number of experiments. It is well known that lead shows 

 an abnormal absorption for soft 7 rays, and the heating- 

 effect observed is no doubt due partly to the absorption 

 of the more penetrating j3 rays and partly to the soft 7 

 rays. The heating effect observed for 7 rays is in rea- 

 sonable accord with the value calculated from the ioniza- 

 tion, and indicates that the underlying assumption is not 

 much in error. Since the ionization observed for 7 rays is 

 mainly, if not entirely, due to the liberation of /3 rays from 

 the matter which the 7 rays traverse, it seems probable that 

 the ionization method can be used with confidence to estimate 

 also the energy of the /5 rays. 



In 1910 Pettersson* made a number of careful observa- 

 tions by balance methods of the heating effect of /3 and 7 

 rays from a radium preparation. The heating effect of the 

 radium preparation was 116'4 when the rays were absorbed 

 in 4 mm. of lead, and 114*5 when the lead was replaced by 

 aluminium 2 mm. thick. The rays in both cases passed 

 through absorbing material equivalent to 4 mm. of aluminium 

 before entering the lead or aluminium cylinder. From the 

 measurement of ionization, it is clear that the difference is 

 due not to the heating effect of the /3 rays as assumed by 

 Pettersson, but mainly to the absorption of the 7 rays by the 

 lead. It seems certain that nearly all the energy of the 

 J3 rays is absorbed in traversing aluminium 4 mm. thick. 



* Pettersson, Ark.f. Mat. Astr. och Fysik, vi. p. 26, July 1910. 



