Febbuaey 1, 1895.] 



KNOWLEDGE, 



39 



To the Editor of Knowledge. 



Dear Sir, — M. Deslandres, in his letter on the electric 

 origin of the chremosphere published in the December 

 number of Knowledge, alludes to my experiments on the 

 radiation of sodium vapour, and mentions as a possible 

 objection to my conclusions the fact that the recipient I 

 used is either attacked by the sodium or is rendered 

 slightly porous when strongly heated, and may allow gases 

 from the Bunsen flame to find their way by diffusion into 

 the sodium vapour, thus setting up chemical reactions 

 whence the D radiation may be derived. He also suggests 

 that ordinary sodium is not in a sufiiciently pure condition 

 for the purpose of my inquiry, since it contains hydrogen 

 in some qiiantity. 



I am very glad of the opportunity he has thus afforded 

 me to answer these objections and state more clearly the 

 nature of the evidence on which I rely to prove that the 

 radiation observed is not the result of any kind of chemical 

 action but is simply due to heat. 



With regard, then, to the possible effect of leakage or 

 diffusion inwards of gases outside the heated tube, I may 

 point out that as hydrogen diffuses through red-hot iron 

 much faster than oxygen or any other gas capable of 

 reacting with sodium, there will always be a stream of 

 hydrogen molecules flowing outwards when hydrogen 

 or coal gas is the medium employed in which to heat 

 the sodium. This outward current is aided in most 

 of my experiments by maintaining a slight pressure 

 inside the tube above the atmospheric pressure. Thus, 

 if external gases gain access to the sodium at all, it 

 can only be ! y diffusion through the outward flowing 

 hydrogen. 



But, even admitting the possibility of minute traces of 

 such external gases finding an entrance to the tube, the 

 chemical action so produced could under no circumstances 

 be expected to originate the broad D emission line or band 

 actually seen ; at the most it would give rise to a fine 

 double D line similar to that of a flame tinted with a salt 

 of sodium, and where the density of the reacting molecules 

 is very small. Fm-thermore, the bright D band corresponds 

 exactly in width with the absorption band seen when white 

 light is passed through the vapour, indicating that every 

 molecule concerned in the absorption is also concerned in 

 the radiation — that is, every fi-ee sodium molecule in the 

 hot part of the tube takes part in the production of the 

 bright D band ; and it would be absurd, imder the circum- 

 stances, to suppose that every molecule was at the same 

 time undergoing chemical change. The only alternative is 

 to suppose that the radiation is the direct result of the 

 heat. 



As to the question of the purity of the sodium employed 

 in the experiments the same argument applies, for there 

 must always be a large excess of free sodium molecules 

 over any others existing as impurities. It is true that 

 sodium will absorb two hundred and thirty-seven times its 

 volume of hydrogen at a certain temperature, but the 

 compound so formed is a very feeble one, and is dissociated 

 again at a temperature far below that at which the vapour 

 begins to glow. 



With regard to the application of my results to the 

 chromosphere radiation, I will only go so far as to claim 

 to have removed one of the chief obstacles in the way of 

 accepting the "incandescent" theory by showing that 

 gases can be made to emit their characteristic light by heat 

 alone ; and I think, therefore, that this theory deserves to 

 be carefully reconsidered by physicists, particularly as 

 it requires fewest assumptions. 



The great difficulty to my mind in accepting M. 

 Deslandres' view is that it necessitates the assumption of 



a continuous discharge of electricity over the entire solar 

 sphere. This seems to imply a shell of matter external to 

 the chromosphere charged m an opposite sense to the 

 photosphere, the difference of potential between photosphere 

 and external shell being in some unknown way maintained 

 constant in spite of the perpetual sort of brush discharge 

 going on. 



The analogy drawn by M. Deslandres between the 

 electrical phenomena of the solar and terrestrial atmos- 

 pheres is a very interesting and suggestive one ; but 

 (fortunately, perhaps, for our well-being) the difference 

 of potential between the higher and lower strata of our 

 air does not necessarily imply a constant discharge of 

 electricity over the entire sm-face of the globe, and there is 

 no phenomenon knowai on the earth, except possibly the 

 aurora, to complete the analogy, which thus seems to fail 

 in its application to the chromosphere radiation. 



.J. Evershed. 



THE PEOGRESS OF ASTRONOMICAL PHOTOGRAPHY. 



To the Editor of Knowledge. 



Dear Sir, — I am pleased to have the opportunity, which 

 Miss Agnes M. Gierke's letter in the September number of 

 Knowledge under the above heading gives me, of frankly 

 admitting the shortcomings of my address, and my regret. 

 But at the beginning of my address, and again near the 

 close, I endeavoured to make its incompleteness known. 

 Thus I said (page 2), " In this brief outline I shall have 

 little more than time to mark the stepping-stones in that 

 onward march ; to trace the details would take volumes." 

 And again (page 23), " In this brief outline of what photo- 

 graphy has done, and is doing, much has been omitted for 

 want of space,'' and I may add that when I began to 

 collect the facts, I thought I should have no difficulty in 

 putting within the limits of my address a fairly complete 

 outline of the stcpa by which astronomical photography has 

 risen to its present importance, but I found I was mistaken, 

 and obUged to leave out many important facts that I had 

 collected about the more recent work. Some others were 

 accidentally omitted ; amongst these was a reference to 

 Prof. Barnard's valuable steps of progress. It was his 

 work on the Milky Way, in 1889, that suggested to me to 

 take up similar work here, and this was acknowledged at 

 my first publication of that work. Other facts have come 

 to my knowledge since. For instance, I have learned that 

 it was his early success as an amateur in astronomical 

 photography, that gave to the world, in the person of 

 Dr. Gill, one of the leading astronomers of to-day ; for 

 it was that which induced him to accept Lord Lindsay's 

 offer and adopt astronomy as a profession. Keference to 

 Dr. Gill's more recent work, which has had so much 

 influence in accelerating the progress of astronomical 

 photography, was quite unintentionally omitted. 



H. C. KUSSELL. 



Observatory, Sydney, 



25th November, 1894. 



Scintcr Notts. 



Summarizing the very carefully compiled and systema- 

 tized facts detailed in the last annual report of the United 

 States Bureau of Ethnology, Mr. J. W. Powell, the 

 director, concludes that tattooing, or face painting, is still, 

 or was very recently, resorted to in various parts of the 

 world for many purposes besides the specific object of 

 tribal, clan, or family designation, and also apart from 



