206 M. J. L. Soret on some Phenomena of 



This publication of M. Hirn's induces me to make known a 

 few of the results I have obtained in observing the polarization 



of flame in general presents no trace of polarization. As is known, it was 

 this observation that served to place one of the first and most important 

 landmarks of the theory of the sun, by informing us that the solar light 

 emanates from a gas, and not from a liquid or a solid. 



" When we have to do with a homogeneous flame, such as that of strongly 

 compressed hydrogen burning in compressed oxygen, when the light ema- 

 nates from all parts of the incandescent gas itself, the fact discovered by 

 Arago exhibits nothing unusual, nothing inconsistent with other known 

 facts. It is not so when the flame is heterogeneous, formed of a real mix- 

 ture of a gas with the dust of a solid body — dust which is far from being, 

 as has often been erroneously advanced, in a state of infinitesimal division ; 

 in a word, it is not so with nine tenths of ordinary flames, the brightness 

 of which can only be explained by Davy's theory. Here, in fact, not only 

 does each of the solid particles emit light of its own, but it must reflect 

 light from other sources ; for it is illumined by the other particles. Then 

 how is the total absence of polarization to be explained? The interpreta- 

 tion, it seems, is not difficult in the case of carburetted gases. 



" Of all known substances, carbon (at least in its most usual state) is 

 that which reflects light the least. Lampblack, for example, reflects very 

 little or none at all ; now, in flame, it is precisely in the form of lampblack 

 that carbon is found, although incandescent. The absence of light polarized 

 by reflection seems therefore very natural in this particular case. In an 

 experiment, however, of which I shall speak subsequently, I have ascer- 

 tained that the smoke of carbon, when produced in a very hot atmosphere 

 and strongly illuminated, appears not black, but of dazzling whiteness. 

 The above interpretation is therefore not so correct as it seems ; and we 

 shall see besides that it cannot be applied in its entirety in a great number 

 of other cases. I cite as striking examples the two following : — 



" 1. I have examined with the Arago-lune polariscope the flame of phos- 

 phorus burning in shade or in bright sunshine, and I have not been able 

 to perceive the least appearance of coloration in either of the two images. 

 When, on the contrary, I directed the instrument towards the vapour of 

 phosphoric acid strongly illuminated by the sun, the coloration became 

 manifest. 



" 2, I have examined in the same manner the lofty and bright flame that 

 issued from the top of a cupola coke-furnace, supplied with air by an ener- 

 getic blast, which served for melting cast iron. Neither at night, nor in 

 bright sunshine have I been able to perceive in it the slightest trace of po- 

 larization. Now, although here also the combustible was carbon, the light 

 of the flame (a very bright light yellow) was certainly not due solely to par- 

 ticles of carbon precipitated in the burning gas ; instead of not solely, it 

 would be perhaps more correct to say not at all. When, at the end of 

 the operation, the furnace-door was opened for drawing out the scoriae, the 

 calcareous flux, &c, a thick bluish smoke (various metallic oxides or salts) 

 escaped from the top instead of the flame. This smoke, strongly illumi- 

 nated by the still-glowing interior, gave to the polarimeter two images as 

 much coloured as those obtained on looking at any light whatever, reflected 

 under the most favourable angle, through an unsilvered glass plate. It 

 is evidently to this vapour of metallic oxides or salts that the flame owed 

 the greater part or even the totality of its brightness while the furnace was 

 in action. 



" How is it to be accounted for that phosphoric acid and the vapour of 



