Heating Effect of lladium and its Emanation. 313 



necessary consequence of the emission of a rays, and was 

 approximately a measure of the kinetic energy of the expelled 

 ol particles. If this were the case, all radioactive substances 

 should emit heat in amount proportional to the energy of 

 their own radiations absorbed by the active matter or the 

 envelope surrounding them. This general conclusion has 

 been indirectly confirmed by measurements of the heating 

 effect of a number of radioactive substances. Duane * showed 

 that the heating effect of a preparation of polonium was of 

 about the value to be expected from the energy of the 

 a particles emitted, while the experiments of Pegram and 

 IVebb t on thorium and of Poole % on pitchblende showed 

 that the heat emission in these cases was of about the 

 magnitude to be expected theoretically from their activity. 



It is of great interest to settle definitely whether the heat 

 of radium and other radioactive substances is a direct 

 measure of the energy of the absorbed radiations. Since 

 the emission of the radiations accompanies the trans- 

 formation of the atoms, it is not a priori impossible that, 

 quite apart from the energy emitted in the form of a, ft, or 

 7 rays, heat may be emitted or absorbed in consequence of 

 the rearrangements of the constituents to form new atoms. 



The recent proof by Greiger and Nuttall § that there appears 

 to be a definite relation between the period of transformation 

 of a substance and the velocity of expulsion of its u particles, 

 suggests the possibility that the heating effect of any a-ray 

 product might not after all be a measure of the energy of the 

 expelled a particles. For example, it might be supposed 

 that the slower velocity of expulsion of the a particle from a 

 long-period product might be due to a slow and long- 

 continued loss of energy by radiation from the a particle 

 before it escaped from the atom. If this were the case, it 

 might be expected that the total heating effect of an a-ray 

 product might prove considerably greater than the energy of 

 the expelled « particles. 



In order to throw light on these points, experiments have 

 been made to determine as accurately as possible: — 



(1) The distribution of the heating effect amongst its three 

 quick-period products, radium A, radium B, and radium C 



(2) The heating effect of the radium emanation. 



* Duane, Comptes JRendus, cxlviii. p. 1665 (1909). 

 t Pegram and Webb, Phvs. Rev. xxvii. p. 18 (1908). 

 X Poole, Phil. Mag. xix. p. 314 (1910). 



§ Geiger and Nuttall, Phil. Mag. xxii. p. 613 (1911) ; xxiii. p. 439 

 (1912); xxiv.p. 647 (1912). 



Phil Mag. S. 6. Vol. 25. No. 146. Feb. 1913. Y 



