NA TURE 



[May 12, 1904 



LETTERS TO THE EDITOR. 

 [1 lie Editor docs not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is tahen of anonymous communications.] 



The Life-history of Radium. 



In a letter under the above title in Nature of May 5, Mr. 

 Whetham brings forward some results dealing with the 

 hypothesis that radium is being produced from uranium. 

 May I be permitted to state that I have been engaged 

 during the last twelve months in an e.vperimental examin- 

 ation of this hypothesis? In the paper in which the sugges- 

 tion was made that radium may be being formed by the 

 disintegration of a parent element possessing heavier atomic 

 weight (Rutherford and Soddy, Phil. Mag., May, 1903, 

 p. 587), this sentence occurs : — " The point is under experi- 

 mental investigation by one of us, and a fuller discussion 

 is reserved until later." Mr. Whetham's letter makes it 

 desirable that the results that have been obtained during the 

 past year should be published. 



Twelve months ago I purilied a kilogram of uranium 

 nitrate until the quantity of radium present was less than 

 10-'^ gram. This was the limit of detection by means of 

 the electroscope employed, using the maximum or equi- 

 librium amount of accumulated radium emanation as the 

 test for the presence of radium. It was arrived at by direct 

 comparisons with the emanation from a standard milligram 

 of radium bromide, by subdivision until its presence could 

 no longer be detected. Unfortunately, owing to the large 

 amount of radium in the laboratory, subsequently intro- 

 duced for the purpose of the helium research, the electro- 

 scopes have been affected, and it is not possible at the 

 present time to be sure of such minute effects as originally. 

 But it may be stated that less than 10- " gram of radium 

 has accumulated in the kilogram of uranium during the 

 past twelve months. I his practically settles the question 

 so far as the production of radium from uranium is con- 

 cerned. 



In a paper read recently before the Royal Society by Sir 

 William Ramsay and myself, an experimental determin- 

 ation of the rate of change of radium was given. It was 

 shown that rather less than one-thousandth part changes 

 per year. The rate of change of uranium may be taken 

 as a million times slower, since its radio-activity is a 

 million times less ; so that, in one kilogram of uranium 

 nitrate, about 5x10-' gram would change per year. The 

 quantity of radium produced was less than lo-" gram, so 

 that the conclusion is arrived at that if uranium changes 

 into radium, less than one-ten thousandth part of the theo- 

 retical quantity is produced during the first year's 

 accumulation. 



The result, of course, may be explained by assuming the 

 existence of intermediate forms between uranium and 

 radium. But from a general consideration of the whole 

 question from the point of view of the disintegration theory, 

 several such hypothetical forms, each with an extended life, 

 must be assumed. So that unless modifications are made 

 in the theory, which at present are not justifiable, the 

 evidence may be taken as indicating that uranium is not 

 the parent element of radium. The experiments will be 

 continued from year to year with the kilogram of uranium 

 nitrate. But as I am leaving England immediately, and 

 shall be away several months, I take the opportunity of 

 presenting the results of the unfinished research, and hope 

 at a later date to give a fuller account. 



Frf.dk. Soddv. 



University College, Gower Street, W.C. 



In their communication to the Royal Society of .'\pril 

 28, Sir William Ramsay and Mr. Soddy by direct measure- 

 ments determine the rate of decay of radium as one 

 thousandth of the mass per annum, giving as the average 

 life of the radium atom about one thousand years. 



This rapid rate of decay, of course, renders if quite out 



NO. 1802, VOL 70] 



of the question to assume that in the radium now existing 

 on the earth we are dealing with the residue of a larger 

 quantity reduced by decay to its present amount. If we 

 carry backwards so great a rate of change we, in fact, 

 arrive at the existence of such large amounts quite a few 

 thousand years ago as to postulate a red hot earth almost 

 within historical times. We are thus either compelled to 

 assume that the rate of transformation observed does not 

 apply generally to terrestrial radium, but only to radium 

 separated by chemical treatment from pitchblende, or that 

 the existing store of radium is derived bv steady supply 

 from some substance of greater atomic weight. The first 

 hypothesis, in view of what is known as to the intimately 

 atomic nature of radio-activity, may be dismissed. 



That the probable source of radium is uranium is advo- 

 cated by Prof. Rutherford in his book on radio-activity. 

 From a conversation with Sir Oliver Lodge I gather that 

 he also considers this not improbable. The reasons for it 

 need not be given here. 



Now if radium is derived from pitchblende, the rate of 

 change of radiujii is a measure of the rate of change of 

 pitchblende, supposing a steady state of supply and loss 

 has been attained. This last condition I think we are 

 entitled to assume, although doubtless from the mathe- 

 matician's point of view a perfect equality would be im- 

 probable. But I will quote Prof. Rutherford (" Radio- 

 activity," p. 334): — "Since radium has a short life com- 

 pared with that of uranium the amount of radium produced 

 should reach a maximum after a few thousand years when 

 the rate of production of fresh radium — which is also a 

 measure of the rate of change of uranium — balances the rate 

 of change of that product." 



Let us now assume as an approximation that from 1000 

 kilos, of uranium the yield of radium under the most favour- 

 able conditions would be one decigram. It may here be 

 observed that the fact of pitchblendes varying in their con- 

 tent of radium is only what is to be expected under the 

 conditions of preservation of the ore, exposed as it is to 

 chemical attack, or, as Prof. Rutherford points out, to the 

 action of percolating water. We have in seeking to learn 

 the content of radium for our present purpose to take the 

 maximum observed. 



The one decigram of radium transforms into sub- 

 stances of lesser atomic weight at the rate of one-tenth 

 milligram per annum. Now this is also the annual 

 supply from 1000 kilos, of uranium. In other words, the 

 uranium breaks down at the rate of i/io'° part of its mass 

 per annum. The average life of the uranium atom is 

 according to this ten thousand million years. 



In determining this average life from so short a period 

 of observation we, of course, make the assumption that 

 the death rate observed is an average one, and that a steady 

 state is attained truly founded on the mean longevity of a 

 vast number of individuals of varying ages, varying rates 

 of loss of corpuscular temperature as well as of varying 

 amounts of initial corpuscular energy, such conditions as 

 would attend material evolution according to Prof. J. J. 

 Thomson's fascinating book " Electricity and Matter." 

 Similar assumptions must be made before we could deduce 

 the average longevity of a vast population from a short 

 period of observation of the death-rate. 



On these assumptions an interval of time is indicated 

 which may be considered a minor limit to the antiquity of 

 matter in our part of the universe. For if the average life 

 is really 10" years, must we not assume that some of the 

 atoms now expiring as uranium were existing ten thousand 

 million years ago? Geological time, as we guess it, is but 

 little more than a moment in the being of so great an era — 

 as thirty-six seconds is to an hour. 



Whether we will ever be able to obtain direct proof of so 

 remote an antiquity is impossible now to say, but it is 

 remarkable that the rate of change of thorium to thorium X 

 affords the same average longevity for the atom of thorium 

 as we arrive at on the data above for uranium, or again 

 from the known rate of change of uranium to uranium X. 

 Thus Rutherford gives 10- " to lo-'' as the change-rate 

 per second to thorium X. The change-rate 10- " for a 

 year's disintegration will be found to lie between these 

 limits. J. Joi.v. 



Trinity College, Dublin, May i. 



