Boltwood — Radio-activity of Uranium Minerals. 271 



solution* containing as much as ten grams of a pure, soluble 

 uraninite was too small to be measured or even to be detected 

 with certainty. This result could be explained neither by the 

 fact that the life of the actinium emanation was too short 

 (3*9 seconds) nor on the assumption that the amount produced 

 was too small, since certain dry preparations obtained from 

 smaller quantities of the mineral and introduced in place of 

 the solution gave out amounts of actinium emanation which 

 could be readily measured. It appeared to be due rather to a 

 very marked retention of the emanation by the solution, the 

 real cause of which has not yet been discovered. It was 

 evident, however, that no reliable data as to the relative 

 amounts of radium and actinium present could be obtained 

 by this method. 



Considerable time was spent in testing the method depend- 

 ing on the measurement of the rise of the activity of the 

 residues obtained from the solutions of the mineral, and the 

 conclusion was finally reached that this was capable at best of 

 giving but very rough approximations for the values of the 

 relative activities. Efforts were then made to devise experi- 

 ments by which more direct and accurate values could be 

 obtained. 



The important discovery by Bragg and Kleemanf that 

 radium and its immediate products (radium emanation, radium 

 A and radium C) emitted a particles with different ranges and 

 velocities, did not seem to justify the earlier assumption made 

 by Rutherford^: that each change which gave rise to a rays 

 supplied about an equal fraction of the total activity, and 

 made a further experimental investigation of this matter 

 very desirable. This was undertaken and the results obtained 

 have been already published. § It was found that the relative 

 activities of the different products were approximately propor- 

 tional to the ranges of the a particles emitted by each product. 



It soon became apparent that the uranium minerals (namely, 

 the primary uraninites) which were most suitable in all other 

 respects to the purposes of the investigation, contained small 

 proportions of thorium. The primary activity of thorium 

 was still in question and the statements made by Hofmann, 

 Zerban, Baskerville and others suggested that thorium might 

 not be a radio-active element. It therefore became necessary 

 to investigate this matter also before any definite conclusions 

 could be drawn from the activities of the minerals. The care- 

 ful examination of a considerable number of minerals contain- 



* A strong current of air was drawn through the solution into a small 

 electroscope. 



fPhil. Mag., x, 318, 1905. 



X Radio-activity, 1st edition, p. 308. 



§ Boltwood, this Journal, xxi, 409, 1906. 



