41 Bolt wood — Salts of Radi um.. 



obtained by the evaporation of a solution from which all eman- 

 ation had been removed. The activity of the freshly prepared 

 residue from this solution was found to be 25 per cent of the 

 activity of the same residue twenty-one days later. 



In none of the papers mentioned are the conditions of exper- 

 iment shown to be such as to preclude the escape of a portion 

 of the emanation from the material tested, nor do they afford 

 any data on the proportion of the total emanation which was 

 retained by the solid radium compound. Experiments con- 

 ducted by the writer on uranium minerals* have demonstrated 

 that it is possible for this class of radium compounds' to lose at 

 ordinary temperatures very considerable proportions of the 

 radium emanation produced within them. 



Since the numerical value of the ratio of the activity of 

 radium itself to the activity of its disintegration products is of 

 considerable importance, and since, moreover, a knowledge of 

 the value of this ratio was essential for the interpretation of 

 other more complicated relations, the following experiments 

 were undertaken in order to determine the relative a-ray 

 activity of radium salts from which all emanation had been 

 removed and the activity of the same salts when the total, 

 equilibrium quantity of radium emanation was retained within 

 them. 



Activity of the Salts. 



The radium salt used was a weak preparation of radium- 

 barium chloride, having an activity not over 100 times that of 

 uranium. The salt had been prepared about six months pre- 

 viously by chemical operations and repeated recrystallization 

 of the chloride with the object not so much of obtaining a 

 strongly active material as of separating any actinium or polon- 

 ium which might have been present in the raw material. A 

 few milligrams of this salt were dissolved in 250 cc of distilled 

 water to which a few drops of dilute hydrochloric acid had 

 been added. Of this solution exactly 10 cc were introduced 

 into a glass bulb by means of a standard pipette, the solution 

 was diluted to about 100°° with water, and the bulb was sealed 

 by fusion. After about sixty days the emanation and other 

 gases in the solution were removed by boiling and were intro- 

 duced into an air-tight electroscope. The leak after three 

 hours, as indicated by the fall of the gold leaf, was equal 

 to -±-60 scale-divisions per minute. This corresponded to a 

 quantity of radium in solution in the bulb equal to 8*5 X 10 -6 

 milligram, or to a content of 8'5XlO~ 7 milligram of radium in 

 l cc of the original solution. 



A number of very thin films of the radium salt were now 

 prepared by slowly evaporating to dryness 10 oc of the standard 



*Ibid. (6), is, 599, 1905. 



