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



If we employ the Bengel burner with its glass chimney, the 

 flame becomes more brilliant, it ceases to be smoky, and the 

 trace of the luminous pencil is no longer perceived ; but if an 

 excess of gas be admitted, so as to cause the flame to shoot, it 

 presents in its upper part the same characters as when the burner 

 was without its chimney. 



With a butterfly, burner, where the combustion is complete, 

 no luminous trace is to be observed, and consequently no 

 polarization. 



If we examine with a polariscope a gas-flame, or better that 

 of an oil-lamp, which shoots, without the addition of solar light, 

 we find that the column of smoke emits light polarized in a ho- 

 rizontal plane. 



In short, for what concerns the experimental results, it is 

 seen that I am in accord with M. Hirn : where the flame burns 

 with vivid brilliance the trace of a luminous pencil is not ap- 

 parent. 



M. Hirn (with some reserve, it is true) explains this by the 

 weakening of the reflecting-power, and by the transparence of 

 the carbon particles at a high temperature. I should not ven- 

 ture to express an absolute opinion on this question, which 

 I have not specially studied ; but before admitting this hypo- 

 thesis, which is partly in disagreement with the principle of 

 the identity of the emissive and absorbing-powers, I ask myself 

 if the absence of polarization in the parts of the flame where 

 combustion is complete cannot be accounted for by the two 

 following considerations : — 



1st. A gas*flame may in general be regarded as formed of 

 several layers. In the centre of the sheet the gas is at a tem- 

 perature inferior to that of the decomposition of the carburets 

 of hydrogen ; it is only on the surfaces of this sheet that decom- 

 position is effected and carbon precipitated in incandescent par- 

 ticles which are burned as rapidly as they are produced. The 

 incandescent layer is therefore very thin, the particles are rare, 

 and the phenomena of reflection and polarization must be in- 

 sensible. 



The fact of the apparent continuity of the flame cannot be 

 alleged as a proof of the abundance of the particles ; irradiation 

 and the persistence of the impressions on the retina sufficiently 

 account for it. Besides, in speaking of the rarity of the par- 

 ticles I only mean a relative rarity. Observing with a lens the 

 feeble luminous trace produced by a solar ray in very clear 

 water suffices for the recognition of the presence of myriads of 

 heterogeneous particles floating in the liquid; and yet the power 

 of illumination is very feeble. 



If, on the contrary, the flame is smoky, the afflux of oxygen 



