628 Miss J. M. W. Slater on the 



The small heating effect of the /3 and y rays compared with 

 that due to the a rays is thus in agreement with calculation 

 based on the relative amount of ionization produced by the 

 rays. The results of this investigation combined with those 

 given in our previous paper show that the heating effect of 

 radium is largely due to the bombardment by the a, particles 

 expelled from its own mass. 



McGiil University, Montreal. 

 Feb. 22nd, 1905. 



Note. — Since the above paper was sent off for publication, a 

 paper by Paschen has appeared (Phys. Zeit. vi. p. 97, Feb. 

 1905), in which he states that the conclusions advanced in his 

 earlier paper have not been confirmed by later work. The 

 ice calorimeter was found to be quite unsuitable to measure 

 such small quantities of heat with accuracy. 



LIX. On the Excited Activity of Thorium. By Miss J. M. 

 W. Slater, B.Sc. (Lond.), Batliurst Student > JSeumliam 

 College, Cambridge*. 



[Plate VIII.] 



Introduction. 



THE following paper consists of two parts, giving an 

 account of an investigation into the effect on thorium- 

 excited activity, first of the cathode-ray discharge, and 

 secondly of heat. The work was undertaken at the sugges- 

 tion of Prof. J. J. Thomson. It has been shown that the 

 activity imparted to a surface which has been in contact 

 with the thorium emanation is probably due to a material 

 deposit. The theory of successive changes as the cause of 

 radioactivity has been worked out by Prof. Rutherford and 

 Mr. Soddy f, who show that the disintegration of the thorium 

 atom gives rise in succession to thorium X, emanation, and 

 an active deposit, the latter being obtained on surfaces in 

 contact with the emanation, and imparting to them a tem- 

 porary activity. This decays to half value in about eleven 

 hours, with the final production of inactive substances in 

 quantities too small to be detected. The active deposit can 

 be dissolved off from the surface by various acids &c, and 

 separated again from the solution by electrolytic and other 



* Communicated by Prof. J. J. Thomson, 

 t Phil. Mag. May 1903. 



