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XXIX. The Relation betiveen Uranium and Radium. — V. 

 By Fkedeeick Soddy, M.A., F.E.S. 



IN the last paper on this subject (IV., Phil. Mag. Dec. 

 1909, p. 846) details were given of the measurements 

 o£ the quantities o£ radium in the three uranium solutions, 

 purified by Mr. T. D. Mackenzie some years ago. It was 

 shown that, within the supposed error of measurement, the 

 growth of radium was proceeding at a rate proportional to 

 the square of the time, and the period of the long-lived 

 intermediate parent of radium (ionium) was deduced from 

 this to be 18,500 years in the case of one solution and 26,000 

 years in that of another. Subsequent measurements have 

 not confirmed these conclusions, and the former apparent 

 rate of growth of radium according to the square of the time 

 has not been maintained in any of the three solutions. A 

 recent redetermination of the constant of the instrument with 

 two of the former standards has shown that it has changed 

 appreciably, the electroscope having become apparently more 

 sensitive. The " constant "f is now 5*2, whereas the last 

 calibration tests, carried out over a period between 350 and 

 250 days previously, had given the mean value of 5' 78. 

 Such a change was not anticipated, as neither the electro- 

 scope nor its reading microscope has been in any way moved 

 or altered in adjustment since first set up, and both have 

 been kept exclusively for these measurements. 



It is very difficult to get a method of checking the sensi- 

 tiveness of an instrument of this character accurately, which 

 shall be quite free from uncertainty. It is necessary to use 

 a standard solution containing a minute amount of radium, 

 and, as is well known (Mme. Curie, Le Radium, 1910, vii. 

 p. 65), these tend to change with time, the amount of radium 

 apparently diminishing, very possibly through the solution 

 of a precipitating constituent from the glass. For fear of 

 this I made some of my standards with very minute quantities 

 of radium (as low as 5 X 10 -11 gram). It is unlikely, how- 

 ever, that the quantity of radium could increase, as it would 

 have to do in this case to explain the change of the constant 

 of the instrument. In the same paper, however, Mme. Curie 

 called attention to variations amounting to 5 per cent, or 

 more in the estimation of radium by means of the emanation 

 when periods of accumulation longer than 48 hours are 

 employed. Some other indications of a possible "radium X " 



* Communicated by the Author. 



f The constant is the number of units of radium (10- 12 gram) required 

 to produce unit leak, i. e. one division a minute. 



