318 Mr. F. Soddy and Miss Ruth Pirret on the 



is not constant in the three minerals. Bat pitchblende and 

 thorianite are so near that the difference can scarcely be 

 regarded as significant. It must be remembered that the 

 figures for pitchblende depend upon the accuracy of the 

 uranium estimation in the standards prepared previous to 

 this work, for the purpose of calibrating the electroscope. 

 There is no reason to doubt the accuracy of these analyses, 

 but the results call for further measurements with other 

 samples of both minerals before the matter can be considered 

 settled. The specimen of thorianite selected was the purest,- - 

 that is to say, the one containing the least foreign matter 

 insoluble in nitric acid, — of many which have been examined. 

 But its uranium content is unusually high, and the specimen 

 in this respect cannot be considered representative. One 

 curious circumstance should be recorded. In the paper 

 immediately preceding this it is shown that the constant of 

 the electroscope has changed apparently since these pitch- 

 blende standards were first prepared. This change was 

 actually discovered in the course of the present work. The 

 ratio of radium to uranium in thorianite appeared, using 

 the original value of the constant of the instrument, to be 

 about 18 per cent, higher than for pitchblende, which is 

 almost exactly what Mile. Gleditsch found. But a redeter- 

 mination with the old standards brought to light the change 

 of sensitiveness of the electroscope ; and although the cor- 

 rected ratio is still higher for thorianite than for pitchblende, 

 the difference is so small that it cannot be accepted without 

 further confirmation. The point at is-ue is rather a fine 

 one, and calls for a degree of accuracy not easily obtained in 

 such measurements. 



As regards autunite, however, the variation in the ratio 

 for the specimen examined (of Portuguese origin) is far 

 greater than that found by Mile. Gleditsch for French 

 autunite. The Portuguese autunite has little more than half 

 the radium in the French autunite. The natural explanation 

 of these results is that the mineral is of so recent formation 

 that the uranium-radium series is not yet in equilibrium. 

 In the preceding paper it is shown that the period of ionium 

 is at least 35,000 years ; and if the autunite examined were 

 of very recent formation, this result is to be expected. The 

 extremely recent formation of autunite is indirectly borne 

 out by the work of Marckwald and Keetman (Chem. Ber. 

 1908, xli. p. 49), who could not find any lead in a crystal 

 of autunite, though '01 per cent, could have been detected. 

 Recently J. A. Gray (Phil. Mag. 1909, xviii. pp. 816 and 

 937) has estimated the amount of lead in autunite spectro- 

 scopically to be only of the order of '005 per cent. Assuming 



