32 



NA TURE 



[November 13, 1902 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opinions ex- 

 pressed 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.] 



Refractivities of the Elements. 



In Nature for October 16 I drew attention to the relation 

 which exists between the refractivities of the inert gases of the 

 atmosphere and that of hydrogen. Further comparison with 

 the values obtained for other elements shows that the occurrence 

 of simple ratios between the refractivities of allied elements is 

 so frequent as to reduce greatly the possibility that they may be 

 due to chance. 



Thus, in the table previously published, there was a gap 

 between krypton and xenon to be filled by an element the 

 refractivity of which should be four times that of hydrogen. This 

 condition is exactly fulfilled by mercury, the vapour of which is 

 also monatomic. The refractivities of chlorine, bromine and 

 iodine are almost exactly in the ratio of 2, 3, and 5, correspond- 

 ing to those of argon, krypton and xenon ; and it is remarkable 

 that the latter trio occupy places in the periodic table which are 

 adjacent to those of the former trio respectively. I cannot find 

 that the value of the refractivity of fluorine has yet been directly 

 determined ; but, if there is any law connecting these figures, 

 it should probably bear the same relation to that of chlorine 

 which the refractivity of neon bears to that of argon, i.e. \. 

 It should, therefore, be equal to ^|-4 = o - i92 (11 = 0139). 



Again, making allowance for the density of sulphur vapour 

 (96), the refractivity of sulphur is to that of oxygen as 2 is to 1. 

 ■o-The following are the figures : — 



The values for Hg, S, P and As were published by Le Roux 

 in iS6t and do not appear to have been verified since. At 

 least, no other determinations are published either by Dufet or 

 by Landolt and Bbrnstein. Iodine shows anomalous dispersion, 

 and the choice of the value 1 -920, which represents the refractive 

 index of the least refracted rays, is arbitrary. 



The values for N, P and As do not fit into the scheme, and 

 a redetermination of them would be interesting. 



Clive Cuthbertson. 



9 York Terrace, N.W., November 3. 



1 Ramsay and Travers, Phil. Trans., cxcvii., A. 1901. 



2 Le Roux, Ann. Ch. et de Ph., Ixi., p. 385, 1861. 



3 Mascart, from Dufet,," Recueil des Donnees numeriques," i., p. 75. 



4 Hurion, Ann. de t'Ecole Normalt, sup. ( 2 e serie), t. vi., p. Sa, 1877. 



NO. I/24, VOL. 67] 



Artificial Mineral Waters. 

 I thank you for your kindly notice of my little book in your issue 

 of October 16 (vol. lxvi. p. 602), and I am quite content to leave 

 your reviewer's remarks concerning its blemishes to the judgment 

 of your readers with the one exception of that dealing with the 

 precautions for preventing the contamination of the carbonic 

 acid gas with ammonia. If your reviewer will call to mind the 

 fact that in the generating vessel there is a mixture with an 

 alkaline reaction until the charge is exhausted, he will not con- 

 sider it as so very astonishing that ammonia may pass into the 

 gasholder. At all events, manufacturers of mineral waters have 

 suffered too much in time past from the presence of gaseous 

 impurities in the carbonic acid gas to permit them to allow the 

 smallest trace of such impurities to contaminate the waters. 

 The conditions of manufacture are such as not to warrant the 

 expectation that either the alkali or the acid in the generator 

 will suffice to hold back traces of either acid or alkaline gases. 



William Kirkby. 



I understand that Mr. Kirkby objects to the statement I 

 made, in my recent review of his book on " The Evolution of 

 Artificial Mineral Waters," to the effect that precautions to 

 avoid the contamination of the carbonic acid gas with ammonia 

 derived from such traces of ammonium salts as might exist in the 

 sodium bicarbonate employed were unnecessary. In reference to 

 this I would point out that sodium bicarbonate does not decom- 

 pose ammonium salts under the conditions in question, and that 

 any tendency to become converted into the normal carbonate 

 owing to rise of temperature is effectually checked by the con- 

 stant production of carbonic acid gas in the liquid in the 

 generator. This is what I meant by saying that the acid used 

 constitutes a sufficient precaution, and if Mr. Kirkby will try 

 the experiment, as I have done, he will find that no trace of 

 ammonia passes from the generating vessel. That manufacturers 

 of foods and beverages should take every possible precaution to 

 avoid the contamination of their products is, of course, highly 

 desirable, but any precautions specially taken for the purpose of 

 avoiding the presence fof this particular impurity are, I still 

 maintain, quite unnecessary. The Reviewer. 



Light-Therapeutics. 



As a constant reader of your valuable and interesting paper I 

 shall esteem it a favour if any of your scientific correspondents 

 can inform me what is the action of the red rays of light on the 

 hair, and what authority is there for supposing that they have a 

 beneficial effect on the scalp. 



In what periodicals, &c, could I find reference to this 

 question ? P. H. Baily. 



Leadenhall House, London, E.C., November 6. 



Waste of Energy from a Moving Electron. 

 In my last week's letter, I observe some corrections are 

 required. Equation (11); the depth of the shell should be 

 vdt [1 - (ujv) cos 6\. Equation (13) ; insert the factor (1 - u-/v ! ) 

 on the right side. Equation (14); divide the second term on 

 the right by R. Oliver Heaviside. 



BRITISH ASSOCIATION GEOLOGICAL 

 PHOTOGRAPHS. 



PROBABLY no instrument — not including the bicycle 

 — has more facilitated the labours of the geologist 

 than the photographic camera, which has for some time 

 past become almost as necessary apart of his outfit as the 

 indispensable hammer. Professional and amateur workers 

 alike carry it, and photographs of geological features 

 do increasingly abound. This was already true in 1SS8, 

 when the happy idea occurred to Mr. Osmund W. Jeffs 

 of forming a public collection of geological photographs, 

 which should be lodged in some central and readily 

 accessible place. As he rightly pointed out, "photo- 

 graphic records of sections and other geological features 

 . . . are not only invaluable aids to geological instruc- 

 tion, but serve also to preserve for future reference the 

 details of many exposures of strata and other landscape 

 features, which in course of time . . . are in danger of 



