July 25, 1907] 



NA TURE 



29- 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he iDulerluke 

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

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications.] 



The Origin of Radium. 



In roplving to Mr. Soddv's communication publi-^hfd in 

 Natirf. of June 13 (p. 1501 I cannot refrain from express- 

 ing my regret on learning that he has apparently taken 

 mv paper in the .[mcrican journal of Science for December, 

 iQo6, as such a serious " criticism " and " imputation " 

 on my part in dealing with his paper on " The Produc- 

 tion of Radium from Uranium." It was my intention 

 merely to point out certain conditions of experiment which 

 appeared to me to be quite essential to the solving of the 

 problem of the growth of radium in uranium compounds, 

 conditions which had apparently been neglected in his own 

 c'Xperimonis, and to show that where these conditions had 

 been fulfilled the results were not in agreement with those 

 which had been obtained bv him. I was then, and am 

 still, of the opinion that the experimental procedure which 

 Mr. Soddv adopted was not suited to give conclusive results 

 of either a positive or negative character, and this opinion 

 is certainly strengthened by the fact that, under very 

 similar conditions of experiment. Prof. Rutherford was 

 unable (Bakerian Lecture, Phil. Trans., 204. 218) to 

 observe the growth of radium in a solution of Giesel's 

 "emanating substance," although he has since concluded 

 ihat radium was actually being produced in quite notable 

 quantities. 



My suggestion of the " accidental and unconscious intro- 

 duction " of radium salts during Mr. Soddy's tests was 

 only one of a number of possible sources of error to which 

 I have already directed attention. That the other sugges- 

 tions have not met with his disapproval, and that he is at 

 least convinced of the necessity of starting with purified 

 uranium salts, would seem probable from his statement 

 that he is now continuing the investigation with purified 

 uranium compounds. I am very glad to learn that the 

 results which he h.as .now obtained entirely confirm and 

 extend the results which I have already published. It 

 may be of interest to add that my original solution of one 

 hundred grams of purified uranium nitrate has recently 

 been tested and found, after a period of more than two 

 and one-half years, to contain less than 10- " gram of 

 radium. 



" The experiments described in this paper are considered 

 to indicate that the results obtained by Mr. .Soddy are 

 without significance and that one or more products of a 

 slow rate of change intervene between uranium and 

 radium." This is the particular paragraph to which Mr. 

 Soddy now raises an objection. I fully realise that this 

 statement is open to criticism ; it was an unsuccessful 

 effort at brevity. .A longer but more satisfactory summary 

 would perhaps have been : — The results of the experi- 

 ments described in this paper are in support of the hypo- 

 thesis that one or more products having a slow rate of 

 change intervene between uranium and radium, and in- 

 dicate that the results obtained by Mr. Soddy are without 

 quantitative significance in so far as they relate to the 

 production of radium by uranium. 



" Commercial salts " of uranium may contain, and 

 usually do contain, quite appreciable amounts of every 

 constituent of the minerals from which they have been 

 prepared. The presence in such salts of a small proportion 

 of the immediate parent substance from which r.adium 

 is derived is therefore in itself no indication of any genetic 

 connection whatever between uranium and radium. My 

 observation of the growth of radium in actinium prepar- 

 ations, even if it has served no other useful purpose, has 

 certainly indicated where the immediate parent of radium 

 is to be sought. To judge from the results which I have 

 obtained in recent experiments along the same lines it 

 would appear that, unless the rate of disintegration of 

 radium now assumed is ijreatlv in error, the chemical 

 Drocess outlined in mv " Note on the Production of 

 Radium from .Actinium " is canable of effecting the essen- 



tially quantitative separation of the radium parent from 

 most of the othei substances present in a uranium mineral. 



In conclusion, it may be desirable to direct attention to 

 the fact that the only evidence we now have that radium 

 is a disintegration product of uranium is the constancy of 

 the ratio between the quantities of these two elements in the 

 natural minerals, a relation which was first pointed out in 

 these columns by the writer. Bertram B. Boltwood. 



Vale L'niversity, New Haven, Conn., June 29. 



The "Double Drift" Theory of Star Motions. 



I HAVE been greatly interested in Mr. Eddington's 

 account in N.\tvre of July 11 (p. 248) of Prof. J. C. 

 Kapteyn's investigations of this subject. .-Xlthough I, do 

 not quite follow his argument for the existence of two 

 overlapping systems of stars (more dramatically termed 

 " two Universes " by Prof. Turner), I yet venture to 

 suggest an e.xplanation of the apparently (perhaps really) 

 opposite " drifts," which seems to me to agree sufficiently 

 with the observed facts. 



If we adopt Lord Kelvin's postulate of a single vast 

 stellar universe very slowly condensing towards its common 

 centre . of gravity, we might expect that the component 

 stars would move for the most part in ellipses or spirals 

 of very varying degrees of eccentricity and of inclination 

 to the mean orbit — perhaps indicated by the Milky Way. 

 If wc further postulate (what is very generally admitted) 

 that our sun is situated towards the central rather than 

 towards the outer portion of the whole system, then, just 

 as the planets, through differential angular motions as 

 regards the earth, appear sometimes to move in a' retro- 

 grade direction or to be quite stationary, so a certain 

 proportion of the stars might be expected, at any given 

 period, to exhibit the same phenomena. 



But further, considering the enormous distances that are 

 known to separate the stars and star-groups from each 

 other and the extreme slowness of their angular motions, 

 there seems no reason why their respective orbits should 

 not be almost as frequently in a right-hand as in a left- 

 hand direction in regard to the central plane of general 

 motion. 



Our knowledge of the actual motions of the stars may 

 not inaptly be compared to what astronomers would possess 

 of the .solar system supposihg the whole of their observ- 

 ations had been limited to a period of about twenty-four 

 hours, arid that the sun was invisible. The motions of the 

 planets and their satellites thus determined would seem 

 as strange and incomprehensible as do those of the stars 

 at the present time, our accurate observations of which 

 have been limited to a few centuries. 



It will probably be of interest to many of your readers 

 (as it certainly w^ill be to myself) if some of your mathe- 

 matical coi-respondents will explain why, and in what way, 

 some such system as is here suggested is incompatible with 

 the facts set forth bv Prof. Kapteyn and others. 



.\lfred R. \Vall.\ce. 



In the article to which Dr. .\. R. Wallace refers, and 

 elsewhere, I have confined myself to attempting to establish 

 the result that the stars distribute themselves into two. 

 systems according to their motions, abstaining as far as 

 possible from defining what physical connection is implied 

 by the rather vague word " system." Whether the twe 

 systems are comparatively permanent and have come 

 together from different parts of space, or whether they 

 may have been evolved from a single system, is, in the 

 present state of our knowledge, a somewhat speculative 

 question, and it is with some reluctance that I enter upon 

 it. Still, without asserting that the hypothesis of two 

 permanent systems is the only possible one, I know at 

 present of no other satisfactory explanation. In the 

 system suggested bv Dr. Wallace (in which the stars move 

 about the centre of the universe in ellipses, some forward 

 and some retrograde, with all sorts of eccentricities) the 

 motions would be for our purposes haphazard. Thus the 

 system would form a single and not a double drift ; the 

 extremely eccentric orbits form a perfect transition between 

 the direct and retrograde orbits. To account for two' 

 drifts, it is not sufficient to show that some stars move 

 forward and some backward ; it must be shown that there 

 is a concentration of the motions about two definite .veloci- 



NO. tq6q. vot. 76] 



