Jan. 6, 1876] 



NATURE 



189 



bility by itself, afterwards made by B. Stewart ; and so I 

 failed to perceive that a soda flame which emits bright 

 D must on that very account absorb light of the same 

 refrangibility. 



" When Foucault, whom I met at dinner at Dr. Neil 

 Amott's, when he came to receive the Copley Medal in 1855, 

 told me of his discovery of the absorption and emission of 

 D by a voltaic arc, I was greatly struck with it. But though 

 I had pictured to my mind the possibility of emitting and 

 absorbing light of the same refrangibility by the mechanism 

 of a system of piano strings tuned to the same pitch, which 

 would, if struck, give out a particular note, or would take 

 it up from the air at the expense of the aerial vibrations, I 

 did not think of the extension of Prevost's theory, after- 

 wards discovered by Stewart, nor perceive that the emis- 

 sion of light of definite refrangibility' necessitated (and 

 not merely permitted) absorption of light of the same 

 refrangibility. 



" Re\-iewing my then thoughts by the light of our pre- 

 sent knowledge, I see that my error lay in the erroneous 

 chemical assumption that sodium could not be free in the 

 flame of a spirit-lamp ; I faUed to perceive the extension 

 of Prevost's theor)-, which would have come in conflict 

 with that error. — Yours sincerely, 



(Signed) " G. G. Stokes 



« To Chas. Whitmell, Esq." 



" P.S., Dec. 31. — As Sir \Vm. Thomson has referred in 

 print to a conversation I had long ago with him on the 

 subject, I take the opportunity of describing my recollec- 

 tion of the matter. 



" I mentioned to him the perfect coincidence of bright 

 and dark D, and a part at least of the reasons I had for 

 attributing the latter to the vapour of sodium, using I 

 think the djTiamical illustration of the piano strings, I 

 mentioned also, on the authority of Sir David Brewster, 

 another case of coincidence (as was then supposed, though 

 it has since been shown to be only a casual near agree- 

 ment) of a series of bright lines in an artificial source of 

 light tt-ith dark lines in the solar spectrum, from which it 

 appeared to follow that potassium was present in the 

 sun's atmosphere. On hearing this Thomson said some- 

 thing to this effect : ' Oh then, the way to find what sub- 

 stances are present in the sun and stars is to find what 

 substances give bright lines coincident with the dark 

 lines of those bodies.' I thought he was generalising too 

 fast ; for thcwgh some dark lines might thus be accounted 

 for, I was disposed to think that the greater part of the 

 non-terrestrial lines of the solar spectrum were due to the 

 vapours of compound bodies existing in the higher and 

 comparatively cool regions of the sun's atmosphere, and 

 having (as we know is the case with peroxide of nitrogen 

 and other coloured gases) the power of selective absorp- 

 tion changing rapidly and apparently capriciously with 

 the refrangibility of the light. 



"If (as I take for granted) Sir William Thomson is right 

 as to the date [1852] when he began to introduce the sub- 

 ject into his lectures at Glasgow (Address at the Edin- 

 burgh Meeting of the British Association [1871], page 

 xcv.), he must be mistaken as to the time when I tsdked 

 with him about Foucault's discovery, for I feel sure I did 

 not know it till 1855. Besides, when I heard it firom 

 Foucault's mouth, it fell in completely .with my previous 

 thoughts. 



" I have never attempted to claim for myself any part 

 of KirchhofFs admirable discover)-, and cannot help think- 

 ing that some of my friends have been over zealous in my 

 cause. As, however, my name has frequently appeared 

 in print in connection with it, I have been induced to put 

 on paper a statement of the views I entertained and 

 talked about, though without publishing. 



" In ascribing to Stewart the discovery of the exten- 

 sion of Prevost's law of exchanges, I do not forget that 

 it was re-discovered by Kirchhoff", who, indeed, was the 

 first \o publish it in relation to light, though the transition 



from radiant heat to light is so obvious that it could 

 hardly fail to have been made, as in fact it was made, 

 by Stewart himself (see * Proceedings of the Royal 

 Society,' vol x, p. 385). Nor do I forget that it is to 

 Kirchhoff that we owe the admirable application of this 

 extended law to the lines of the solar spectrum." 



SCIENCE IN THE ARGENTINE REPUBLIC* 



THE fourth part of the Bulletin of the National Aca- 

 demy of Sciences recently founded at Cordova, in 

 the Argentine Republic, completes the first volume of this 

 remarkable work, of which we have previously given some 

 account to our readers.f The present part is mainly 

 occupied by the conclusion of a long article upon the 

 vegetation of the httle known province of Tucuman, in 

 the interior of the Republic, by Dr. Hieronymus, com- 

 menced in a former number. This is based upon the 

 observations made by the author during a long and ex- 

 tensive scientific journey in that province, and upon the 

 collections amassed by Dr. Lorentz in the same districts 

 in 1871 and 1872, which have been mainly determined by 

 Prof. Grisebach, of Gottingen. A second important 

 article is by Dr. D. A. Doring, and treats of the land and 

 freshwater Molluscs of the Argentine Republic, amongst 

 which are a considerable number of new species, and 

 several interesting novel forms discovered by the author. 

 A third memoir, from the pen of Dr. Burmeisterj treats 

 of the abnormal HjTnenopterous insects of the Linnean 

 genus Mutillay and forms a complete monograph of the 

 native species of this group, which will be very acceptable 

 to entomologists. By the chronicle appended to the 

 number, we leam that the strife which has prevailed 

 between the Director of the Academy and the six German 

 professors originally imported for its constitution has ter- 

 minated in the signal defeat of the latter. After the 

 expulsion of about half the number, the remainder re- 

 signed, and their places have been filled by other pro- 

 fessors from the same country, whom we trust Dr. 

 Burraeister will find more tractable. That they are full 

 of work is evident by the contributions to science already 

 pubhshed in the present volume, upon the successftJ 

 completion of which we heartily congratulate the ener- 

 getic and illustrious Director of the Academy of Natural 

 Sciences of the Argentine Republic 



SOME UNSOLVED PROBLEMS IN THE 

 MANAGEMENT OF THE MARINE AQUA- 

 RIUM 



TT would be fatal to further progress in that direc- 

 -*■ tion in which so much has been achieved during 

 the last ten years, if the zoological conditions of even 

 the most successfiil of existing marine aquaria were 

 to be bUndly accepted as incapable of improvement, and 

 especially if further experiment in reference to the vexed 

 question of aeration were to be barred by the assumption 

 that any one of those rival systems which are typified in 

 the practice of Brighton, Sydenham, or any other similar 

 establishment, is necessarily the best which can be 

 attained. 



More discussion than it has yet received is due to the 

 broad question whether the total or almost total exclusion 

 of vegetation fi-om public aquaria is based on necessity or 

 philosophy ; whether artificial may not be advantageously 

 supplemented by this most natural and automatic mode 

 of aeration ; and the further question remains, to what 

 extent must the conditions of the aquarium be modified, as 

 regards circulation and introduction of air, in order to 

 render practicable the establishment and maintenance of 

 a healthy vegetation, if the propriety of its introduction 



* " Boletin de la Acadeaiia Nacional de Ciendas exactas existente en U 

 Universidad de CordoTa." Eotr^a iv. (Buemos Ayrcs, 1875.) 

 t See Naturk, toL to. p. 253. 



