240 Intelligence and Miscellaneous Articles. 



These experiments confirm the theory of the reversal of the lines 

 by absorption ; for the condition of success is the bringing together, 

 as completely as possible, of the carbons, consequently the formation 

 of a large quantity of vapour in a very small space. In the centre 

 the temperature is very high, the radiations very intense ; the cor- 

 responding lines are at the same time bright and widened ; around 

 the central focus the layers are cooler, they emit radiations less in- 

 tense, but clearer as the length of an undulation. Thus the reversal 

 line is very fine when the thickness is sufficient to produce absorp- 

 tion ; it would become wider and wider, as happens in the case of 

 soda, if the temperature were still more raised. 



I will add a few words in order to set forth again the connexion 

 between these experiments and spectrum- observations of the sun. 



When Kirchhoff gave the explanation of the reversal of the lines, 

 his view was admitted, that the dark lines of the solar spectrum 

 were due to a continuous atmosphere enveloping the sun and ab- 

 sorbing certain radiations of the photosphere. Nevertheless, in the 

 chemical point of view, the constitution of this atmosphere, in which 

 would be found pell-mell the vapours of so many different bodies, 

 presents some difficulties ; besides, its existence is contradicted by 

 the comparative observations of the margin and the centre of the 

 sun. If the absorbent atmosphere had a sensible thickness, the ab- 

 sorption-spectrum would vary with the thickness traversed by the 

 rays which reach us, and consequently would have a different aspect 

 at the centre and at the margin of the solar disk. We know that 

 it is not so ; and hence astronomers have concluded that the emis- 

 sion of luminous radiations, and the absorption of certain of them, 

 both take place on the photosphere itself. This hypothesis is veri- 

 fied by our experiments, and even precisely defined to a certain 

 degree ; for they show : — 



1st. That a very slight thickness of vapour can produce the re- 

 versal of the lines — a thickness absolutely imperceptible at our dis- 

 tance from the sun ; and 



2ndly. That there is no advantage in supposing a continuous at- 

 mosphere, however thin, the absorption being all local, and produced 

 spontaneously by the external cooling about each incandescent point. 



It will moreover be remarked that the experiment described above 

 is a genuine reproduction of the hypothetical constitution of the sun, 

 and a synthesis of the spectrum-phenomena presented by it, — the 

 incandescent carbon, on which is the metal, playing approximately 

 the part of the photosphere, with a layer of vapours above at a very 

 high temperature and emitting radiations which are partially absorbed 

 by the exterior layer. 



In short, a new fact brought out by these experiments is the re- 

 versal of a great number of metallic lines : hitherto success had 

 been obtained in reversing scarcely any besides the sodium-line. 

 Moreover they confirm the observations (which at first appeared 

 very singular) of the partial reversal of a group of lines, by showing 

 that this apparent singularity is the general case. And, lastly, they 

 justify, and even precisely define, certain hypotheses concerning the 

 constitution of the sun. — Comptes Bcndus, July 21, 1871. 



