212 Prof. F. Soddy and Miss A. F. R. Hitchins on the 



be kept discharged during the waiting interval without 

 changing its constant. This is a point of considerable im- 

 portance in measurements of radium by the emanation 

 method, and had long ago been adopted in all other measure- 

 ments by this method; but, in order to preserve continuity, 

 the old method had been retained with all the measurements 

 by the old instrument in this set of experiments. With the 

 new electroscope the new plan was adopted. Very great 

 care was taken to avoid any introduction of error by this 

 change, and measurements taken with the same standards 

 by the old and new magnifying-power methods agreed 

 perfectly. In addition, the magnifying power of the new r 

 microscope was considerably less than that of the old, and 

 the two changes together caused a reduction of the sensitive- 

 ness of the new instrument to about one-third of that of the 

 old. The accuracy of the measurements, however, was not 

 affected by these changes. 



In former papers, somewhat different values have been 

 employed for the ratio of radium to uranium in pitchblende, 

 and all the former results have been recalculated to the same 

 value, viz. 3*4 x 10 ~ 7 g. radium per gram of uranium. In 

 the present work, a large number of new standards from 

 carefully analysed uranium minerals were prepared, as it 

 was found that the old standards, prepared in 1909, no longer 

 agreed among themselves. As is well known, it is practically 

 impossible to keep such standards indefinitely, owing to the 

 tendency of part of the infinitesimal amount of radium 

 present to precipitate out of solution. 



In the following three tables are given particulars of the 

 calibrations of the instruments. The first refers to the 

 original instrument, the second to the new instrument, and 

 the third to the latter after an accident to the gold leaf. 

 The uranium minerals used were those employed in an 

 earlier research*, on the ratio of radium to uranium in 

 minerals. The radium-barium chloride preparations used 

 were some containing about 10 mg. of radium (element) per 

 kilogram, in which the radium has been determined by 7-ray 

 measurements of spherical samples against a radium standard, 

 according to the method described in *' Chemistry of the 

 Radio-Elements/ Part I. Second edition, 1915, p. 93, in which 

 the absorption of the 7-rays in the preparation itself is 

 corrected for. Known weights of these were dissolved, and 

 the solutions diluted to convenient strength. The electro- 

 scope was thus calibrated independently on the 7-ray standard 



* F. Soddy and Miss R. Pirret, Phil. Mag. [6] xx. p. 345 (1910) : 

 xxi. p. 652 (1911). 



