192 TEANSACTIOXS OF THE SOCIETY OF 



and solicited by mutual forces of attraction and repulsion ; and he gives tlie dif- 

 ferential equations, whose integration would furnish the value of the molecular 

 displacements. By a method peculiar to himself, M. Galopin, with the help of 

 certain artifices of analysis, arrives at the equation of the velocities of luminous 

 waves already given by Fresnel, an equation which may be regarded as repre- 

 senting a surface called that of elasticity, and from which he deduces the equation 

 of the surlixce itself of these waves. Besides the two surfaces in question, there 

 still exist six others, the study of which enables him to arrive rapidly at the 

 properties of the surftice of the waves, at those which concern the peculiar points 

 and planes of that surface, the conic and cylindrical refractions, and finally the 

 ordinary and extraordinary rays of crystals of two axes. 



M. Lucien de la Rive has made researches on the differences of density of a 

 gaseous mass revolving around an axis in a cylindrical vessel, and has arrived 

 at a formula which enables him to calculate these different densities. The dif- 

 ferences in question are little appreciable for volumes of gas and dimensions of 

 vessels of inconsiderable quantity ; but they increase in proportion as the diame- 

 ter of the vessels is larger. The last named physicist read a memoir on the 

 conductibility of ice for heat. After having given the detail of his experiments, 

 he recapitulates them mathematically, and proceeds to deduce to the value of 

 the co-efficient h, of the conductibility of ice, a co-efficient which he finds =0.25, 

 that of glass being 0.13, and that of porcelain 0.24. Then applying his results 

 to the formation of ice on a surface of water below 0°, he seeks for the law 

 according to which this formation takes place, and he arrives at three equations 

 corresponding to three difi"erent epochs of the formation in question. Now, as 

 the last of these equations is that of a parabola, he thence deduces that after the 

 lapse of quite a few days of frost, the ice can only increase very slowly. The 

 author finally establishes the agreement of his theory with known fiicts, with 

 the observations of Flauguergues particularly, and concludes by indicating 

 briefly the application which may be made of it to the formation of the polar 

 ices. 



Verbal reports. — Professors Wartmann and Marcet,in several successive verbal 

 communications, brought to the notice of the society the interesting discussion 

 in progress between MM. Magnus and Tyndall, on the absorption of heat by 

 gases. M. Tyndall, in repeating his experiments without employing the dia- 

 phragm of rock-salt, has removed one of the most serious objections of M. Mag- 

 nus. From his results he derives consequences of importance for the theory of 

 Wells on dew, and for other atmospheric phenomena. Professor Marcct made 

 the additional remark that Dulong, in his investigation regarding the specific 

 heat of gases, had set out with the hypothesis that gases do not radiate ; now, 

 since M. Tyndall has established that they do radiate, the results of Dulong 

 would seem to call for revision, account being taken of the radiation. Professor 

 Plantamour occupied our attention with the views of M. Hipp respecting the 

 establishment of electric clocks in cities. Their employment at Geneva has 

 gi'eatly conduced to a determination of the conditions by which their disposal 

 should be governed. Their position in gas-lanterns exposes them to great vari- 

 ations of temperature from summer to winter, to tiie unfavorable influences of -dust 

 and humidity, and to the disturbing effects of the discharges of atmospheric 

 electricity and of concussions produced, whether by gusts of wind or the fre- 

 quent washing of the lanterns — concussions which have sometimes caused tem- 

 porary loss of the current. By reason of these difficulties, M. Hipp would prefer 

 that such clocks should be placed in the wall of the fronts of houses rather than 

 in the gas-lanterns. M. Philip Plantamoiu- presented an analysis of the re- 

 searches of M. Editing on the formation of ice in the northern seas. According 

 to the latter, the sea begins to freeze from the bottom ; the water being there 

 cooled below the point of congelation, the least shock, the passage of a fish for 

 instance, suffices to determine solidification and to produce the sudden formation 



