294 



A^A TURE 



[January 26, 1905 



LETTERS TO THE EDITOR. 

 [The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he undertake 

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

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications .] 



The Origin of Radium. 



Eight months have elapsed since 1 wrote in your 

 columns (Nature, May 12, 1904) giving an account of 

 some experiments designed to test the view advanced 

 by Prof. Rutherford and myself that radium is a product 

 of the radio-active change of uranium. I then stated that 

 in I kilogram of uranium nitrate that had been under 

 observation over a period of one year since it was com- 

 pletely freed from radium, the quantity of radium repro- 

 duced in that time was less than one-ten-thousandth of 

 the quantity theoretically to be e.\pected. This result has 

 been widely quoted, more widely, perhaps, than I intended, 

 for the result was a preliininary conclusion only, and, as 

 I pointed out, obtained under very unfavourable conditions 

 owing to the very powerful preparations of radium that 

 had been in use in the laboratory for other researches. 

 The necessity for publishing it was to a certain extent 

 forced upon me by the attention the problem was beginning 

 to attract from other investigators, and by the prospect 

 of several months' absence abroad. I relied on the fact 

 that the result being negative, the presence of the radium 

 in the laboratory could have had no effect, but in this I 

 was mistaken. 



Since my return I have resumed the research in the 

 new chemical laboratories recently erected here, into which 

 no radium has so far been brought, and have found that 

 the earlier result was affected by an error which invali- 

 dates the conclusion drawn. It is therefore my duty to 

 point this out at once without waiting for any further 

 results. I am now fairly satisfied that there is a steady 

 production of radium from uranium, and although the 

 quantity formed, as measured by the amount of radium 

 emanation evolved, is of a lower order of magnitude than 

 is indicated by the disintegration theory, it is much greater 

 than the ten-thousandth part. 



At the present time, about eighteen months since the 

 commencement of the experiment, the kilogram of uranium 

 nitrate in solution contains, so far as the amount of 

 emanation evolved is a measure, about ijXio-" gram of 

 radium, and if the whole series of measurements from 

 the commencement are re-calculated, eliminating the error 

 alluded to, they are fairly consistent with there having 

 been a steady production of radium at this rate con- 

 tinuously from the commencement. This gives the value 

 2X10-'' for the fraction of the uranium changing per 

 year, whereas the most probable theoretical estimate is 

 10-'. The new result is thus still only one-five-hundredth 

 of the theoretical. 



The error in the result published last May was not in 

 the determination of the amount of radium emanation 

 evolved from the uranium, but in the determination of the 

 amount of emanation given by a known weight of radium, 

 against which the first mentioned determination was com- 

 pared. The measurements on the uranium are in good 

 agreement with those recently obtained, whereas the com- 

 parative experiments with radium gave results too high 

 owing to extraneous radium in the laboratory. For the 

 effect from the uranium is so minute that to obtain a 

 comparable effect with the radium emanation, the quantity 

 of the latter obtainable from the smallest weighable 

 quantity of pure radium bromide must be diluted and sub- 

 divided until only a millionth part at most remains. Thus 

 if any emanation were present in the air of the labor- 

 atory used for the dilution, or if by mischance any of the 

 gas apparatus, rubber tubing, or mercury had been used 

 previously in experiments with powerful radium prepar- 

 ations, the results obtained would be completely false. 

 Tt is now known {vide Rutherford, Phil. Mag., November, 

 1904, p. 637) that even metals, as copper and silver, 

 absorb the radium emanation appreciably and slowly evolve 

 it. The utmost precautions have to be observed in 

 standardising the rate of leak of the electroscope by the 

 emanation from a known weight of radium, so that each 



NO. 1839, VOL. 71] 



successive dilution of the emanation is performed in an 

 entirely new apparatus with new mercury and rubber 

 connections. Otherwise emanation is absorbed from the 

 gas rich in it and given out to the diluted gas, and when 

 the final dilution should contain only one-millionth of the 

 original emanation, as in these experiments, it will be in 

 reality far richer. This explains the apparently para- 

 doxical result I obtained that the determinations of the 

 amount of radium produced were far too low, owing to 

 the extraneous radium of the laboratory. 



The research is being continued with the view of elimin- 

 ating what appears a probable explanation of the too low 

 rate of production. It may be that under the conditions 

 of the experiment the greater part of the emanation is 

 retained by the uranium solution and not evolved as gas. 

 New methods are being tried, and it is hoped that they 

 will give a positive answer to this question. 



Frederick Soddy. 



The University, Glasgow, January 20. 



A New Radio-active Product frona Actinium. 



At the suggestion of Prof. Rutherford, I have made an 

 examination to see if there is any product in actinium 

 corresponding to the product UrX in uranium or ThX in 

 thorium. The investigations were made with a prepar- 

 ation of the emanating substance of Giesel (of activity 

 300 times that of uranium), which has been shown to 

 be identical in radio-active properties with the actinium 

 of Debierne. 



Taking into consideration the similarity of actinium 

 and thorium, both as regards their chemical and radio- 

 active properties, I resolved to try if the method used by 

 Rutherford and Soddy for the separation of ThX would 

 not serve also to separate an analogous product from 

 actinium. The experiments were at once successful. If 

 ammonia was added to a solution of actinium in hydro- 

 chloric acid, the actinium was precipitated, while a small 

 amount of a very active substance was left behind in the 

 filtrate. This substance, which is so similar in properties 

 to ThX, will be called actinium X (AcX). 



The product AcX, immediately after its separation, 

 weight for weight, was more than a hundred times more 

 active than the original actinium. The activity Increased 

 In the first day after removal to about 15 per cent, of 

 its original value, and then decayed with the time accord- 

 ing to an exponential law, falling to half value in about 

 ten days. The actinium from which the AcX had been 

 removed, almost inactive Immediately after separation, 

 gradually recovered its lost activity, .^s in the case of 

 thorium, the curve of recovery of the activity was com- 

 plementary to the curve of decay of AcX. 



The behaviour of the product .\cX is thus completely 

 analogous In all respects to that of ThX, only the constant 

 of change has a different value, which is characteristic 

 for AcX. 



Special experiments, made for the purpose, showed that 

 the emanation was produced- from AcX, and not directly 

 from the actinium. The latter, immediately after separ- 

 ation of AcX, gave off very little emanation, while AcX 

 produces the emanation in large amount. The amount of 

 emanation from AcX diminished with the time at the 

 same rate that AcX loses Its activity. At the same time 

 the actinium gradually increased in emanating power, due 

 to the production of fresh AcX, and finally reached an 

 equilibrium value. 



The product AcX gives out both o and 18 and probably 

 7 rays. It Is, however, difficult to determine whether the 

 k ravs arise directly from .^cX or from the excited activity 

 to which the emanation gives rise. 



There is an interesting point of distinction between the 

 radio-activity of thorium and actinium. After the separ- 

 ation of AcX, the actinium is almost completely inactive, 

 only 4 per cent, of the maximum activity being observed. 

 It is probable that this amount could be still further re- 

 duced by successive precipitations. Thorium and radium, 

 on the other hand, always show a non-separable activity 

 of about 25 per cent, of the maximum. This points to 

 the fact that the activity from ordinary actinium is due 

 entirely to .^cX and Its successive products, and that little. 



