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



2nd. M. Ed. Becquerel has proved that lampblack deposited 

 from the flame of benzine upon a plate of mica, in a layer suffi- 

 ciently thin to be translucent, does not give in the phosphoro- 

 scope any trace of phosphorescence*. Now in general, with 

 solid substances, fluorescence has a certain duration and is con- 

 founded with phosphorescence. 



3rd. If this residual light proceeded from a fluorescence, it 

 ought to be remarked also in the smoke of a flame ; the intensity 

 of the illumination is certainly strong enough for this. Now, as 

 we have said, polarization is complete in this latter case. One 

 can very well conceive that, the particles being relatively much 

 disseminated and distributed throughout the thickness of the 

 incident pencil, the proportion of light undergoing multiple 

 reflections may be insensible, while in the lampblack deposited 

 on a solid substance all the particles are in mutual contact, they 

 are accumulated on the same surface, and must reflect recipro- 

 cally a notable quantity of light. 



In brief, lampblack, whether as a continuous deposit or in the 

 state of corpuscles disseminated in a gas, appears to me to con- 

 stitute a very good type for this kind of phenomena of illumina- 

 tion. Indeed, on the one hand these particles are very tenuous, 

 and, on the other, they are opaque — which eliminates the per- 

 turbations which may be induced by refraction, leaving only 

 what is due to reflection. 



There are other fumes, in the widest sense of the term, which 

 give rise to very different results. For example, the vapour of 

 water precipitated in small quantity in the vesicular state, and 

 illuminated by a pencil of light, gives also phenomena of polar- 

 ization ; but while ordinary vapour reflects under an angle of 

 vision of 90° light polarized in the plane of vision, vesicular 

 vapour in the same conditions diffuses light partially polarized 

 in a perpendicular plane. This is what Professor Tyndall has 

 described in his beautiful researches on the optical properties of 

 cloudy substances \, and I have verified also upon vapour con- 

 densed in divers manners. 



For an angle of vision less than a right angle a diminution of 

 the polarization is first observed, then a neutral point (towards 

 70°) ; at last, under an angle of about 65° and below, polariza- 



the solar rays certain substances, as sulphur and carbon, emit a light which 

 has the closest correspondence with that of the fluorescence of quinine, 

 esculine, &c, except that there is no change of refrangibility : it is what 

 M. Lallemand has called isochromatic fluorescence. 



* M. Ed. Becquerel h«s been so extremely obliging as, at my request, 

 to repeat this experiment in my presence. 



t See Archives des Sciences, 1869, vol. xxxiv. p. 172. 



