138 Messrs. A. S. Russell and F. Soddy on the 



Bearing in mind the fact that the uranium mineral contains two 

 products, uranium X and radium E, which contribute /3-rays 

 with little or no 7-radiation, this result is in agreement with 

 what is to be expected from the determinations just described 

 of the ratio for the three single products, radium C, meso- 

 thorium 2, and thorium D. The thickness of the lead through 

 which the 7-rays were measured also made no practical 

 difference in the ratio. Eve has already shown that the 

 7-rays from thorium nitrate and uraninite have practically 

 the same penetrating power. The similarity of penetrating 

 pow r er of the 7-rays from thorium minerals is to be expected 

 from the result, detailed in Section III., that the radium C 

 7-rays are intermediate in penetrating power between the two 

 types of 7-rays given by the thorium series. 



There remains one practically important question with 

 reference to the 7-rays of the thorium series. What is the 

 relative intensity of the radiation contributed by the two 

 products mesothorium 2 and thorium D? Mr. Alexander 

 Fleck has done some experiments with Ceylon thorite with a 

 view to obtaining information on this point, and although, so 

 far, only preliminary results are available, these maybe given 

 here. The thorite was dissolved in hydrochloric acid, preci- 

 pitated with excess of ammonia, filtered, and the precipitated 

 hydrates subjected to the same treatment five times, in 

 all, without unnecessary lapse of time. The filtrates were 

 collected, evaporated and ignited together, and sealed up in 

 a box with a thin aluminium lid for /3-ray measurement. The 

 filtrate from a sixth precipitation, carried out immediately 

 after the fifth, treated separately in the same way, w r as 

 inactive, showing that by this treatment the whole of the 

 mesothorium and thorium X had been separated. The /3-ray 

 decay curve of the preparation was taken for some weeks. 

 The intensity of the rays rose to a maximum after 2 days (due 

 to thorium 1)) and thtn fell, until a constant minimum, due 

 to mesothorium only, was attained. The decay curve for the 

 maximum onward was extrapolated back to an origin corre- 

 sponding to the time of the last precipitation with ammonia, 

 and from the initial and final value of the intensity of the 

 /3-rays, the proportion due to each product could be obtained. 

 The measurements indicate that the thorium D contributes 

 distinctly more /3-radiation than the mesothorium 2. The 

 difference is not great, and may be estimated provisionally as 

 from 25 to 50 per cent. Since thorium D is richer in /3-rays, 

 relatively to the 7-rays, than mesothorium 2, it follows that 

 the 7-radiation from the two types must be very similar in 

 intensity. This result, although only approximate, will prove 



