Excited Activity of Thorium. 629 



methods (von Lerch*, Peg-ram f); it can be volatilized at a 

 white heat (Gates) J : but no methods hitherto employed 

 have been able to destroy the activity, or to alter its 

 rate of decay ; under nearly all conditions this follows an 

 exponential law, falling to half value in about eleven hours. 

 In a few cases, where electrolytic and chemical methods have 

 given substances with a quicker rate of decay, this can best 

 be explained (as will be shown later) by assuming that there 

 has been a separation of different stages in the decomposition 

 of the active deposit, having different radioactive constants. 



I. Influence of the Cathode Ray Discharge. 



It was thought possible that cathode rays, which penetrate 

 the molecules of substances on which they fall, might be 

 able to affect the internal changes going on in the active 

 molecule, and so alter its rate of decay. 



Method of Investigation. 



The following method was employed. A quantity of 

 thorium hydroxide was placed in an earthed metal box, and 

 the substance to be made active was fixed on an insulating- 

 support over the thorium, and connected to the negative 

 pole of a battery of small storage-cells, the positive pole 

 being earthed. The exposures used were generally about 

 24 hours ; even after this time there was usually a slight 

 initial rise in the activity of the surface after its removal 

 from the thorium, and the active matter was not exposed to 

 the cathode rays until the period of regular fall (half value 

 in 11 hours) had set in. 



The activity was tested by observing the rate of deflexion 

 of a quadrant electrometer in the usual way. The active 

 metal was suspended inside a metal cylinder, from which it 

 was insulated by ebonite slabs and guard-ring, and was con- 

 nected to one pair of quadrants of the electrometer, the other 

 pair being permanently earthed. The cylinder was charged 

 to a potential high enough to give a saturation current across 

 the enclosed air space (200 volts was generally used). In 

 making an observation a key was raised so as to break the 

 contact between the earth and the pair of quadrants con- 

 nected to the active metal, and the time taken for a deflexion 

 is a given length of the scale was observed by means of 

 a stop-watch. 



Two similar active surfaces which had been exposed to 



* Drude's Annaltn. Nov. 1903. t Phys. Review, Dec. 1903, p. 424. 

 | Phys. Review, p. auo (1903). 



