October 



1907J 



NA TURE 



661 



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, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications.] 



Transit of Mercury across the Sun's Disc, November 

 13-14, 1907. 



Premising that the times given on p. 451 of the 

 Nautical Almanac for 1907 are the Greenwich mean astro- 

 nomical times of the several contacts in the above transit 

 as seen from the centre of the earth, it may be useful to 

 the readers of Nature to record the corresponding Green- 

 wich mean astronomical times of the contacts as seen from 

 Greenwich. These times are deduced by the formukie 

 printed on the above-mentioned page of the Nautical 

 Almanac : — 



d. h. m. s. 

 External contact at ingress ... November 13 22 23 24 

 Internal contact at ingress ... ,, 13 22 2() i 



Internal contact at egress ... ,, 14 i 48 10 



E.\ternal contact at egress ... ,, 14 i 50 50 



Angle from north point of sun of contact at ingress, 62° ; 

 angle from north point of sun of contact at egress, 345° ; 

 measured towards the east in both cases. 



A. M. W. Downing. 



H.M. Nautical Almanac Office, October 20. 



Origin of Radium. 



In a letter to Nature (June 6) I gave the experimental 

 evidence which led me to conclude that in ordinary 

 actinium preparations a new substance was present which 

 was slowly transformed into radium. By a chemical 

 method this substance was separated from actinium, and 

 a solution of the latter was obtained which showed no 

 appreciable growth of radium over a period of eighty 

 days. Observations on tliis solution have been continued 

 over a total period of 240 days, and there is still no 

 detectable increase in the quantity of radium. The growth 

 of radium, if it occurs at all, is certainly less than 1/500 

 of that observed in other experiments. 



In two recent letters to Nature (September 26 and 

 October 10) Dr. Boltwood has given the results of his 

 later experiments in this direction. He has confirmed my 

 conclusions, and has, in addition, been successful in 

 devising a satisfactory method of separating this new sub- 

 stance from actinium, and has examined its radio-active 

 and chemical properties. He suggests that the name 

 " ionium " be given to this new body, which is probably 

 the immediate parent of radium. Dr. Boltwood is to be 

 congratulated for his admirable work on this very difficult 

 problem, for, apart from the chemical operations, the radio- 

 active analysis required for correct deduction is unusually 

 complicated and difificult. 



Dr. Boltwood has not been able to separate the parent 

 of radium from actinium by the reagent employed by me, 

 viz. ammonium sulphide, but has found the use of sodium 

 thiosulphate effective. In explanation of this discrepancy, 

 he suggests that I emoloyed old ammonium sulphide. As 

 a matter of fact, I did not use the ordinary laboratory 

 solution of ammonium sulphide, but added ammonia to 

 the actinium solution, and then saturated it with 

 sulphuretted hydrogen. The complete separation effected 

 in my experiment was, I think, probablv due to .in acci- 

 dental production of finely divided sulphur in the solution. 



In a letter to Nature of last wek, Mr. N. R. Campbell 

 raised objections to the name " ionium " given by Dr. 

 Boltwood to the new body, from the point of view that 

 every radio-active substance should be given a name to 

 indicate its position in the scheme of rad'O-acHve changes. 

 This system is very excellent in theory, but I have found 

 it extremely difficult to carry out in practice. The con- 

 tinual discovery of new products in very awkward positions 

 in the radio-active series has made any simple permanent 

 system of nomenclature impossible. Besides uranium and 

 NO. 1983, VOL. 76] 



thorium, twenty-four distinct radio-active substances are 

 now known to exist in radio-active minerals. The number 

 of products still to be discovered is, I think, nearly ex- 

 hausted. When there is a general consensus of opinion 

 that this is the case, I feel it will be very desirable for 

 physicists and chemists to meet together in order to revise 

 the whole system of nomenclature. There is not much 

 to be gained in doing so immediately, as the discovery of 

 a new product in the midst of a series would entail the 

 alteration of the names of a possible half-dozen others 

 which follow it. .\t the same time, I think it w'ill be 

 desirable to retain a distinctive name for those radio-active 

 substances which, like radium, have a long enough life 

 to be separated in sufficient quantity for an examination 

 of properties by the ordinary chemical and physical 

 methods. It is probable that the parent of radium fulfils 

 these conditions, and should thus have a distinctive name 

 like radium. 



Personally, I do not much like the name " ionium," 

 but for similar reasons neither do I care for the name 

 " actinium." It is not easy to suggest a name that is 

 at once simple and explanatory. I have for some time 

 thought that possibly " paradium " or " picradium " might 

 be suitable for the new substance. The former name 

 suggests that it is the parent of radium, but I recognise 

 that a possible play on words may make it unsuitable. 

 The name uranium A, suggested by Mr. Campbell, in itself 

 innocuous, is open to the objection that in the case of 

 radium, thorium, and actinium the suffix A is applied to 

 the first product of the disintegration of the respective 

 emanations, while no such emanation has been observed 

 in the initial series of changes of uranium. 



E. Rutherford. 



University of Manchester, October 27. 



The Nature of X-rays. 



In a paper published in the October number of the 

 Philosophical Magazine (pp. 420-449), Prof. Bragg, after 

 discussing the properties of various electric radiations, 

 arrives at the conclusion that although a beam of X-rays 

 contains some ether pulses, these may not after all con- 

 stitute the bulk of Rontgen radiation. In place of the 

 usually accepted theory, he proposes the hypothesis that 

 these rays consist mainly of " neutral pairs " (consisting 

 of a positive and a negative particle) each revolving in a 

 plane containing its direction of translatory motion. This, 

 he considers, affords an easier explanation of the properties 

 of the rays, and is not improbable a priori. 



I do not intend to discuss more than one point here, for 

 it seems to me that the record of a simple experiment is 

 of more value in deciding between the two hypotheses than 

 a series of comparisons or discussion of probabilities 

 possibly could be. 



To explain the phenomena of secondary radiation from 

 light atoms, he supposes that a " pair " striking a light 

 and yielding atom does not suffer disarrangement, but 

 may be returned unchanged and constitute a scattered 

 ray. He also supposes that it is liable to be taken up 

 only by an atom revolving in the same plane as itself, and 

 that if ejected again the subsequent rotation and transla- 

 tion will continue to take place in the one plane. The 

 secondary radiation in .a direction perpendicular to that of 

 propagation of the primarv will then consist of pairs 

 rotating in the plane of the primary and secondary pro- 

 pagations, and the tertiary will therefore be strongest 

 when in the same plane, thus explaining the polarisation 

 effect. 



It is important to notice th.at this theory can only 

 account for the amount of polarisation which I found to 

 exist in a secondary beam from carbon (Proc. Royal Soc, 

 A, vol. Ixxvii., iQOf^). if the assumed relation between the 

 plane of rotation and direction of propagation is an accurate 

 one. 



Now it can easily be shown that, according to the ether 

 pulse theory, when an unpolarised X-ray beam is incident 

 on a substance of low atomic weight, such as carbon, the 

 intensity of secondary radiation is at a minimum in a 

 direction perpendicular to that of propagation of the 



