282 



NATURE 



[August 25, 1923 



The experimental observation of the surface- 

 opalescence of water presents special difficulties owing 

 to the great ease with which this li<iuici catches dust 

 and grease. The difficulties have, however, been 

 successfully overcome and the effect satisfactorily 

 demonstrated, both with water rendered dust-free in 

 sealed bulbs and with the water- film on a clean block 

 of melting ice kept free of dust by a gentle stream of 

 gas blowing upon it. 



C. V. Raman. 



210 Bowbaz&ar Street, Calcutta, 

 June 28. 



On Continuous Radiation from the Sun. 



Prof. J. Q. Stewart recently published in these 

 columns (Nature, February lo, p. i86) a very 

 interesting communication on the optical and 

 electrical properties of ionised gases. For some time 



East I have been engaged in investigations on similar 

 nes, and I wish to direct attention to one important 

 side-result. It is well known that in estimating the 

 surface temperature of heavenly bodies (as has been 

 done by Coblentz, Abbot, Wilsing and Scheiner, 

 and others), from their continuous spectra, it is 

 always tacitly assumed that they radiate like per- 

 fectly black bodies. Several investigators have 

 pointed out that this assumption does not tally 

 with experimental results. The temperature ob- 

 tained by applying the total radiation law and the 

 method of isochromatics to the spectral - energy 

 curve are at variance with each other. They are 

 also different from the temperatures obtained from 

 the ionisation-theory. 



The best example is afforded by the sun, which, 

 according to the careful measurements of Abbot 

 and Wilsing, shows a spectral-energy curve consider- 

 ably deviating from that of a black body (see E. A. 

 Milne, Phil. Trans, vol. 223, p. 218) ; the fact has 

 been discussed by many investigators, including 

 Schwarzschild, Groot, Milne, Dietzius, and others. 

 There are very weighty reasons why the sun would 

 not radiate like a black body. A black body or a 

 full radiator is one which absorbs all the radiant 

 energy which falls on it, reflecting none. Such an 

 ideal body is nowhere met with in the world, but 

 Wien and Lummer realised it by making use of a 

 hollow enclosure maintained at a constant tempera- 

 ture, and provided with a small hole, the idea being 

 that a beam of radiation within the enclosure would 

 describe an infinitely circuitous path, and what the 

 emission lacks in fullness will be made up by repeated 

 reflections. 



It is clear that none of these conditions is fulfilled 

 in the case of the sun. The surface of the sun 

 contains a large percentage of free electrons, and 

 positive charges, which endow it with a large re- 

 flecting power. This point will be clear if we re- 

 member the analogous case of metals. According 

 to the electromagnetic theory, metals derive their 

 high reflecting power from the presence in them of a 

 large number of free electrons, or rather electrons 

 which are easily excited to vibration by incident 

 radiation. A theory of emissivity of metals on this 

 basis was worked out by Aschkinass in 1905, and has 

 been verified by the experiments of Rubens and Hagen, 

 Langmuir, and others. 



The presence of a large percentage of free electrons 

 on the surface of the sun would, thus, endow it with 

 a high reflecting power. The surface being an open 

 one, the hollow enclosure condition is not realised. 

 Thus the conclusion seems to be irresistible that the 

 total emission from the surface would fall far short 

 of that of full radiator. The form of the spectral- 



NO. 2808, VOL. I 12] 



energy curve suggests the emissivity E* varies as 

 ^— 0(X») where \>x>\, but about this point judg- 

 ment should be reserved now. 



Turning to the stars, it is easy to see that similar 

 conditions would hold. The analogy with metals 

 enables us to say that the emission from low tem- 

 perature stars would fall far short of that from a 

 lull radiator at the same temperature, while for 

 stars with very high temperature, emissivity may 

 approach that of a black body. 



Prof. Eddington's work on the constitution of 

 stars is based on the assumption that inside the stars 

 total emissivity varies as T* ; this assumption is 

 probably not affected, for, inside the stars, the hollow 

 enclosure condition is largely fulfilled. 



Megh Nad Saha. 



University College of Science, Calcutta, 

 July 5- 



Separation of Common Lead into Fractions 

 of Different Density. 



By fractional crystallisation of lead assay foil, about 

 300 grams in all, two end fractions, each weighing 

 about 60 grams, were obtained. These fractions were 

 then purified according to Stas's method. For the 

 density determinations, about ten grams of each was 

 melted in an atmosphere of hydrogen and allowed to 

 solidify in a vacuum. The densities of samples pre- 

 pared in this way were determined in specific gravity 

 bottles. 



Density of lead from crystals end of fractionating 

 series : — 1 1-345 ±0005. 



Density of lead from mother liquor end of fractionat- 

 ing series : — 11-313 4; 0005. 



A sample of Stas lead, which Mr. C. T. Heycock 

 very kindly gave me, was found to have the density 

 11-328 in one experiment and 11-326 after re-melting. 



The difference in density between the above-men- 

 tioned fractions persisted after granulating the metal 

 and also after re-melting the granulated metal under 

 potassium cyanide. It was discovered in the course 

 of these experiments that lead which has soUdified 

 slowly is not homogeneous as regards density, — the 

 parts which freeze first being denser. 



Out of eleven experiments, only one was inconsistent 

 with the view that the original lead had been separated 

 into two fractions which had different densities. 



The work is being continued. 



R. H. Atkinson. 



Goldsmiths' Metallurgical Laboratory, 



University Chemical Laboratory', 



^Cambridge, July 18. 



Proposed International Survey of the Sky. 



On the initiative of the French National Meteoro- 

 logical Service, it has been decided to take photo- 

 graphs of the clouds three times daily during the 

 week September 17-23, inclusive, at as many stations 

 as possible throughout the countries of western 

 Europe. As the number of official meteorological 

 stations is limited, it has been proposed to enlist the 

 services of those professional and amateur photo- 

 graphers who are willing to co-operate voluntarily in 

 the work. The photographs should be taken as nearly 

 as possible at 7 a.m., i p.m., and 6 p.m. G.M.T. (not 

 summer time). The photographer should make a 

 note of the direction in which the camera is pointing 

 when the photograph is taken {e.g. north, south-west, 

 etc.) ; if more than one photograph is taken at any 



