July 22, 1909] 



NA TURE 



97 



shape of even a condensed account of the matters 

 specially selected by Dr. Garrod, and so one need only 

 mention that the anomalies treated at length are 

 albinism, cystmaria, alkaptonuria, and pentosuria. 



It must not be supposed that the examination of 

 these comparatively rare conditions is devoid of 

 general interest, for it is often just these curious acci- 

 dents of perverted tissue change which form the 

 opportunity of the keen observer in unravelling the 

 perple.xities of the normal state. The natural sequence 

 of studv is physiology first, pathology next. But very 

 often an inversion of this order leads to important 

 accessions to knowledge. Dr. Garrod is to be con- 

 gratulated on having been successful in such an 

 experiment, and those interested in metabolism cannot 

 do better than study his lucid and bright exposition of 

 the subject. W. D. H. 



Practical Testing of Gas and Gas-meters. By C. H. 

 Stone. Pp. x+337. (New York : John Wiley and 

 Sons; London : Chapman and Hall, Ltd., 1909.) 

 Price 15^. net. 

 This is a laboriously complete compilation of the 

 various methods of testing gas for illuminating power, 

 purity, chemical composition, and calorific value, and 

 of proving the accuracy of the indications of gas- 

 meters. The subject is one mainly of technical in- 

 terest only, and very specially so even for that, and 

 hardly suitable, therefore, for detailed consideration 

 in these columns. An examination of the book shows 

 how great a diversity of apparatus has been devised 

 and thrust upon the innocent gas-producing world, and 

 how gratuitous some of the diversity is. Where ap- 

 paratus has been designed for ofificial testings, the 

 objection to protecting the manufacture by patents 

 has given the constructor liberty to alter and "im- 

 prove " or spoil an instrument, as the case may be. 

 The American has a great opinion of the English 

 official ten-candle lamp designed by Mr. Harcourt, but 

 he will not take it as he finds it, and so he makes 

 an American pattern. The English official calorimeter, 

 too, judging by the observations made, has also gone 

 through a metamorphosis in crossing the Atlantic. 

 As is to be expected, the book is well got up and illus- 

 trated, and its value is increased by the inclusion of 

 a number of tables of value to those whose business is 

 to test and examine gas. 



A Compendium of Food-microscopy. By E. G. 

 Clayton. With sections on Drugs, Water, and 

 Tobacco. Compiled, with additions and revision, 

 from the late Dr. A. H. Hassall's works on Food. 

 Pp. xxxix+431. (London : Baillifere, Tindall and 

 Cox, 1909.) Price los. 6d. net. 

 This book, written by an ardent disciple of the late 

 Dr. Hassall, is largely based on Dr. Hassall's works on 

 food and its adulteration. An account is given of the 

 microscopical characters of all the principal vegetable 

 food-stuffs, beverages such as tea and coffee, fruit 

 preserves and condiments, and of tobacco, opium, and 

 a few other drugs, together with those of the chief 

 adulterants of these substances. In addition, chapters 

 are devoted to foods of animal origin and the parasites 

 which may infect them, milk, cream, butter, and 

 cheese, and to the microscopical flora and fauna of 

 water. The book is profuselv illustrated with line 

 drawings to scale, which usually reproduce very clearly 

 the characteristics of the substances they are intended 

 to depict, though occasionally there is an irritating 

 want of svstematic arrangement of the figures. Fig. A, 

 for instance, sometimes being on the right, some- 

 tfmes on the left, of the page. Though the botanical 

 terminology is not always that used nowadays, on 

 the whole the book should form a useful addition to 

 the library. of the analyst and microscopist. 



NO. 2073, VOL, 81] 



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 0/ Nature. 

 No notice is taken of anonymous communications.] 



Molecular Scattering and Atmospheric Absorption. 



Since Lord Rayleigh discussed the question of mole- 

 cular scattering, and its bearing on the explanation of the 

 blue colour of the sky, our experimental and observational 

 data have become much more trustworthy. While our 

 knowledge of the number of molecules in a gas allows us 

 now to calculate with sufficient accuracy the amount of 

 direct sunlight which is diverted by scattering, Mr. Abbot's 

 series of measurements at Washington and on Mount 

 Wilson gives us the actual observed opacity of the air 

 for different wave-lengths. 



Lord Rayleigh showed that, on the hypothesis of the 

 elastic solid theory of light, small particles of matter, 

 which act simply by adding inertia to the sether, scatter 

 light and retard the passage of a wave passing over them 

 in such a way that the relation 



/._ 3^'^V-i)'- 

 3N 



holds, where k is the coefficient of extinction of energy, 

 /i the refractive index, and N the number of molecules per 

 cubic centimetre. He showed, further, that the same 

 equation may be deduced from the electromagnetic theory 

 if the particles locally affect the inductive capacity of the 

 medium. In the forthcoming new edition of my " Optics " 

 it will be proved that the equation is independent of 

 theory, provided n is nearly equal to unity ; the limit of its 

 applicability is only reached when there is a retardation of 

 phase at the origin of the scattered light the square of 

 which is appreciable, and it can be shown that this is 

 actually the case except within the region of anomalous 

 dispersion. The range of the formula may be further in- 

 creased if ^(,a''— i)^ is written for (fi—i)'. 



For N I have used Rutherford and Geiger's value 

 272x10", and with the known value of the refractive 

 index of air, 'k may be calculated. If h is the height of 

 the homogeneous atmosphere above the point of observa- 

 tion, e-"' is the fraction of light which would reach 

 the observer if no light were lost in any other way than 

 by molecular scattering. In the following table the trans- 

 mitted light calculated in this manner is compared with 

 Abbot's observed figures. The first column gives the wave- 

 length, the second column contains the observed values 

 of the transmitted energy for Washington, taking all 

 observations into account, while the third column gives 

 the number calculated from the observations on February 

 15, 1907, when the air was exceptionally clear. The calcu- 

 lated values are entered into the fourth column. The last 

 three columns give the corresponding numbers for Mount 

 Wilson. The selected clear day in this case was October 

 II, 1906. 



Waihingtnn Mount Wilson 



. ^- , , ' 



Wave- Observed Observed Calcu- Observed Observed Calcu- 



length mean clear day lated mean clear day la'ed 



4x10-' ... 0-55 ... 072 ... 071 ... 073 •■ 076 ... 076 



5 ... 070 ... 0-84 ... 0-87 ... 0-85 ... 0-89 ... 089 



6 ... 076 ... 0S7 ... 094 ... 089 ... 0-92 ... 0-95 



7 ... 0-84 ... 0'9O ... 0-96 ... o'94 ... 0-96 .. 0-97 



8 ... 0-87 ... o'94 ... 0-98 ... 0-96 ... 0-99 ... 0-98 

 10 ... 0-90 ... 096 ... 099 ... 097 ... 099 ... 099 



The close agreement between the two last columns shows 

 that on a clear day on Mount Wilson atmospheric absorp- 

 tion is practically accounted for by molecular scattering. 

 There is a slight indication of selective absorption in 

 the red, but otherwise the columns are in complete agree- 

 ment. On the average day there seems an additional 

 absorption of about 2 per cent. It is remarkable that, 

 even at Washington, the calculated absorption for blue 

 light should so nearly agree with the calculated value ; 

 this means that even at the sea-level the greater part 

 of the absorption on a clear day is due to scattering 

 by the molecules of air. The large diminution in the 



