38 



NA TURE 



[July 8, 1922 



trons, or to both, and these axes are associated 

 different amounts of energy, it is possible for the mass 

 of the atom to be different for the different positions 

 of the nuclear axis, since by the Theory of Relativity 

 energy possesses mass. That is, the model proposed 

 predicts the existence of isotopy. If the direction 

 of the nuclear axis as described above is determined 

 by the structure of the outer shell of electrons, we 

 should expect the existence of different axes in the 

 same atom to be favoured in those atoms the outer 

 shells of which are complete (inert gases), nearly com- 

 plete (halogens), or just forming (alkali metals). An 

 inspection of Aston 's list of isotopic elements shows 

 that it is in the neighbourhood of the inert gases that 

 the phenomenon of isotopy chiefly occurs. 



W. Hughes. 

 63 Goldington Avenue, Bedford. 



The Intensity of X-ray Reflection from Powdered 

 Crystals. 



In the May number of the Philosophical Magazine 

 which has just reached us, Mr. C. G. Darwin has 

 presented a most valuable discussion of the reflection 

 of X-rays from imperfect crystals. He shows that, 

 on account of the difficulty in determining the effec- 

 tive extinction coefficient of the X-rays in such 

 crystals, it is very difficult to calculate with accuracy 

 the intensity of the reflected beam. Hence he is 

 unable to make a satisfactory comparison between the 

 theoretical formula? and the existing experiments on 

 the intensity of X-ray reflection. This result is in 

 general agreement with the conclusion reached by one 

 of us (Physical Review, July 1917) on the basis of 

 somewhat similar considerations. 



Mr. Darwin concludes that a more satisfactory test 

 might be made on powdered crystals, since in this 

 case the only factor contributing appreciably to the 

 extinction is the ordinary absorption of the X-rays in 

 the powder, which can be measured directly. We had 

 arrived at the same conclusion, and have made 

 quantitative measurements of the intensity of the 

 X-rays scattered by powdered crystals. 



In our most recent experiments, the Ka line from 

 molybdenum (\ + 0-708 A.), after reflection from a 

 crystal of rock-salt, was allowed to fall upon a plate 

 of powdered sodium chloride. The first order reflec- 

 tion from the [100] faces of the powdered crystals 

 then entered the ionisation chamber. The method 

 was thus similar to that employed by W. H. Bragg 

 (Proc. Phys. Soc, Lond., 33, 222, 1921) except that 

 the primary rays were homogeneous. The ratio of 

 the energy reflected into the ionisation chamber due 

 to this first order line to that incident upon the plate 

 was 2-94 x 10" 4 , with a probable error of about 10 

 per cent. The theoretical intensity of the line was 

 calculated from a formula identical in significance 

 with Darwin's formula (10-4) (loc. cit.), except that 

 correction was made for the absorption of the X-rays 

 in the crystal mass. We thus obtained the value 

 2-7 x 10- 4 , which is in satisfactory agreement with the 

 experimental measurement. Thus, at least to a close 

 degree of approximation, the theory of X-ray reflec- 

 tion based upon the classical electrical theory gives 

 accurate results. 



This comparison of theory with experiment may 

 be viewed in another manner. Any formula for the 

 intensity of X-ray reflection must depend upon the 

 value of a function f, the magnitude of which is 

 determined by the distribution of the electrons in the 

 atoms. The theoretical value 2-7 x io~ 4 mentioned 

 above is based upon the value ^ 2 = o-59, i.e. upon the 

 assumption that the intensity is 59 per cent, as great 

 as it would be if all the electrons in sodium and 



NO. 2749, VOL. I IO] 



chlorine were grouped together at the centres of their 

 respective atoms. This value was estimated by one 

 of us (loc. cit.) on the basis of some of W. H. Bragg's 

 measurements of the relative intensity of the different 

 orders of X-ray reflection from rock-salt. The 

 corresponding value of f- as determined by the 

 measurements of W. L. Bragg, James and Bosanquet 

 is 0-43 (Phil. Mag., July 1921). To obtain our 

 experimental value 2-9 xio -1 for the intensity of 

 reflection from powdered crystals, the value of i/< 2 

 must, however, be 0-64. The difference between the 

 latter two values of ^ 2 supports Darwin's suggestion 

 that the method employed by Bragg, James and 

 Bosanquet for studying the intensity of X-ray 

 reflection is not wholly trustworthy. 



We hope in the near future to be able to report 

 experimental results of a considerably higher degree 

 of accuracy than those described above. 



Arthur H. Compton. 

 Newell L. Freeman. 



Washington University, Saint Louis, May 30. 



Discoveries in Tropical Medicine. 



I am much astonished to learn, for the first time, 

 from Dr. L. W. Sambon's letter in Nature of May 

 27, that during the whole period of my work in India 

 (from April 1895 to February 1899) he " was almost 

 daily at Manson's house " and was allowed to read 

 my private letters to Manson and to " discuss every 

 detail." Are we to understand by this that his 

 almost daily visits to Manson's house continued for 

 all this period, and that during it he read all my 

 letters to Manson, numbering no, and averaging a 

 thousand words each in length ; or merely that he 

 read a few of the letters which Manson showed to 

 him from time to time ? Dr. Sambon would appear 

 to claim the former interpretation of his words, 

 because he proceeds to suggest that he is intimately 

 acquainted, in consequence of his knowledge of these 

 letters of mine, with all details concerning the 

 relations between my work and the theories of 

 Manson. If so, I can only say that I am amazed 

 and hurt. Many of my letters to Manson were of a 

 very private nature, and it is difficult for me to 

 believe that he would have handed over the whole 

 of this correspondence without reserve to a gentle- 

 man who was at the time a stranger to me and was 

 in no way concerned with my affairs. 



Moreover, when Manson sent some of my letters 

 to Lord Lister he was, of course, careful to inform me 

 of the fact ; but he never mentioned the name of 

 Dr. Sambon, so far as I remember, in all the fifty-six 

 letters which he wrote me in reply to mine, as surely 

 he would have done had he decided to submit my 

 letters to a third person without my previous consent. 

 Nor did Sir Patrick Manson ever mention this matter 

 to me during the many years which have elapsed 

 since the correspondence referred to ceased. On the 

 other hand, if Dr. Sambon did not see all my letters 

 to Manson, including the private letters, he cannot 

 possibly have that close knowledge of my work which 

 he seems to believe he possesses. 



The remainder of Dr. Sambon's communication 

 in Nature referred to makes me still more doubtful 

 regarding the interpretation which is to be placed 

 upon his words ; for it seems to me that he does not 

 understand the said relations between my work and 

 the theories of Sir Patrick Manson. May I also take 

 the opportunity to state that I for one can scarcely 

 accept as sound any of the conclusions which he has 

 set forth in your columns in the letter referred to. 



Ronald Ross. 



