478 A. S. Eve — Relative Activity of Radium and Thorium. 



In a previous paper* I have proved that the 7-rays from 

 radium and from thorium are absorbed at a precisely equal 

 rate during their passage through lead, whilst the 7-rays from 

 uranium are much more readily absorbed. In the same paper, 

 the ratio of the 7-activities of radium and thorium was also 

 stated, but erroneously, inasmuch as thorium nitrate was used 

 for the determination, and we now know that the thorium was 

 therefore not in radio-active equilibrium. Furthermore, it has 

 been shownf that the observable 7-radiation from radium in 

 equilibrium is due solely to radium C. In the case of thorium 

 it is known that the 7-rays are due to thorium C. Therefore, 

 the 7-radiation of thorium C, which is, in due sequence, a dis- 

 integration product of radio-thorium, is a measure of the radio- 

 thorium present. We should, therefore, expect the 7-radiation 

 of a thorium mineral or salt to be proportioned to the amount 

 of radio-thorium present. So that the results of 7-ray obser- 

 vations ought to be in agreement with those made by the a-rays 

 or by the emanation method. 



In the experiments the gamma-radiation was measured in 

 the usual manner by an electroscope. 



The substances compared were as follows : 



(1) Radium bromide, equivalent to *25 ms as compared with the 



sealed standard at the Physics Building, McGrill Uni- 

 versity. This standard consists of 3*69 mg of radium 

 bromide, which Rutherford found gave per gram a 

 heating effect of 110 gram-calories an hour. 



(2) Thorianite, Ceylon, 454 grams, containing 11 per cent of 



uranium and 79 per cent of Th0 2 . 



(3) Thorium nitrate, prepared by Eimer and Amend, received 



from them a year ago, which contains 47 per cent of 

 ThO, 



The activities of these three substances, measured by their 

 7-radiation, were respectively proportioned to 



(1) 18-5 



(2) 8-34 



(3) 1-38 



These are -the means of several observations under varied 

 conditions, and are the actual divisions per minute observed 

 with the electroscope. 



Specific Activities. 



(1) The activity of the radium bromide, under the conditions 

 of the experiment, was 18'5/*25 per mg. or 74,000 per 

 gram. 



*Phil. Mag., April, 1906. f This Journal, July, 1906. 



