The Production of Radium from Uranium. 769 



practically restricted to these two elements. Considerations 

 analogous to those which indicated helium as one of the 

 ultimate products of radioactive change, pointed strongly to 

 uranium being the parent element of radium and polonium. 

 The latter elements are only to be found in the uranium 

 minerals, and only those richest in the latter contain sufficient 

 of the new elements to make the extraction profitable. Since 

 radium and polonium are transition forms, they do not, like 

 helium, accumulate to an indefinite extent with the lapse of 

 time. In any disintegration series equilibrium is attained, 

 and the relative quantities of the various members of the 

 series do not further alter when certain definite proportions 

 of the transition forms have accumulated. These proportions 

 expre>-ed in terms of the quantity of the parent element are 

 the inverse ratios of the rates of change of the transition 

 forms and parent elements respectively. From the relative 

 radioactivity of uranium and radium, it followed that the rate 

 of change of the former is about one-millionth of that of the 

 latter, so that the quantity of radium accumulating in a 

 mineral should be about one-millionth of the quantity of 

 uranium present, and this ratio should be independent of the 

 age of the mineral. This is approximately the value found 

 experimentally by M. and Mine. Curie, and by Griesel, so 

 that from the first there was a strong presumption in favour 

 of the view that uranium in its own disintegration is con- 

 tinually producing radium. Rutherford has recently (Phil. 

 Mag. [6] viii. 1904. p. 636) brought forward direct experi- 

 mental evidence that polonium is one of the disintegration 

 products of radium ; and in this paper similar direct experi- 

 mental evidence is given to show that the radium itself results 

 from the disintegration of uranium. The continuous dis- 

 integration series thus revealed, starting with the heaviest 

 element known, embraces the majority of the known types of 

 radioactive matter, and although direct experimental evidence 

 i> -till lacking, probably ends, so far as the manifestation of 

 radioactive phenomena is concerned, in the ultimate production 

 of one of the heaviest non-radioactive elements, bismuth or 

 lead. The time occupied in the passage, after the initial 

 change of the uranium until the final change of polonium, 

 during which the changing matter is present as intermediate 

 forms, is a very considerable one, the average time being 

 probably of the order of ten thousand years. 



On this account experiments by a direct method, in which 

 a reproduction of radium is looked for in uranium originally 

 freed from that element, must be continued over a long 



