368 SOCIETY OF PHYSICS AND NATUKAL HISTOEY OF GENEVA. 



times to the riglit, sometimes to the left of the observer. Fui-ther, it may vary 

 in direction in the same body with the tint of the light which traverses it, for we 

 Lave notice of a liquid which is l^evogyral for the rays of one extremity of the 

 spectrum and dextrogyral for those of the other extremity. When Faraday 

 found, in 1845, that The plane of polarization of a ray traversing an inactive 

 substance may be made to turn by placing the substance within the magnetic 

 field, he enhanced in an unexpected manner both the interest and the difficulty 

 of this problem of molecular mechanics. Wiedemann showed that this artificial 

 rotation increases in proportion as the length of wave of the colored ray dimin- f 

 ishes. Verdet ascertained that there exist substances for which the rotation is 

 positive, others for which it is negative, (that of water being taken for unity,) 

 but that it is not necessarily connected W'ith refrangibility. Wertheim deduced 

 from his experiments that in general it is absent in solids endued with double 

 refraction. 



Is rotary polarization owing to an action exerted by the substance which 

 transmits tlie luminous (or calorific) ray, or should it be referred to an influence 

 experienced by the ether which surrounds ajid penetrates matter, properly so 

 called? The 'first of these suppositions was taken into favor when Faraday 

 found that the magnetic rotation is distinguished from the natural, by the very 

 important fact that it augments with the length of the course of the ray, whether 

 direct or reflected, in the transparent medium. 



Professor de la Rive has resumed the study of this subject, and connnunicated 

 to us (7th May) a comprehensive review of his experiments. These have been 

 made with divers solids, among others glass compressed by the fulminating dis- 

 charge of Ruhrakorft''s machine, as well as with different liquids. Our distin- 

 guished colleague had already remarked, {Traife (VElccfricite, t. i, p. 555,) that 

 the phenomenon seems connected in an essential manner with the density more 

 or less considerable of the intcrmolecular ether, and consequently with the refrac- 

 tive power of bodies ; but in his new researches he has found that the density of 

 the body itself exercises a great influence, independently of that of the ether 

 which it includes. Thus, with the electro-magnetic intensity at his disposal, he 

 has ascertained that the rotation being 8° in sulphur of carbon having a density 

 of 1,2G3, it became 16°, that is precisely double in thallic alcohol, a liquid of 

 which the refrangibility is slightly superior, and which possesses a density much 

 more considerable, (3.55.) 



It is known that the rotation of the plane of polarization persists for some 

 instants after the electric current has been interrupted. M. de la Rive has 

 satisfied himself by a great number of experiments in which he has succeeded in 

 measuring the dur'ation of that persistence, and in appreciating the circumstances 

 which influence it, that this effect cannot be explained by the inertia of the pon- 

 derable molecules. It is a consequence of the magnetism remaining in the iron 

 of the electro-magnet, for it no longer takes place when inductive spirals simply, 

 without a metallic nucleus, are employed. M. de la Rive concludes that artificial 

 rotary polarization, although greatly' influenced by the molecular constitution of 

 bodies, is not due to an alteration which magnetism might determine in that con- 

 stitution, but rather to an action exerted indirectly on the ether through the 

 medium of the ponderable particles. This explains why the phenomenon depends 

 at once on the state of the intcrmolecular ether, and on the arrangement of the 

 number and the nature of the particles in a given volume of a body. 



M. Achard recapitulated (April 16) the conclusions of a popular lecture given 

 by INI. Clausius on the second principle of the mechanical theory of heat. Accord- 

 in o- to the last-named savant, the work which natural forces can execute, and 

 which is contained in the movements of the different bodies of the universe, is 

 successively tranformed into heat. This heat seeks incessantly to pass from 

 warmer bo'dies into colder ones, so that there will be gradually established a 

 definitive equilibrium between the heat radiating into ether and that which 



