PHYSICS. 3G1 



a wooden frame. A weighed quantity of pure mercury — generally 220 

 grams — is placed in the inner cylinder. By moving the whole around 

 on its trunnions, this mercury may be poured from one part of the 

 inner cylinder to the other, and the temperature thus equalized. The 

 substance whose specific heat is to be determined is placed in the plati- 

 num cylinder, which is then heated in a special apparatus to the tem- 

 perature of boiling water, transferred rapidly to the calorimeter, and 

 this moved on its trunnions until the temperature ceases to rise. The 

 highest point being noted, the specific heat is easily calculated. The 

 various precautions necessary, and the methods for determining the 

 constants of the instrument, are given in the paper. {Am. Chetn. <7., 

 February, 1881, ii, 361.) 



Wiillner has examined critically the formulas in use for calculating 

 specific heats, especially that portion of these formulas which involve the 

 corrections. He finds that the inexactness of the ordinary formula arises 

 from the fact that it does not take account of the condition that during 

 cooling the calorimeter value is increased by the product of the weight 

 and specific heat of the substance, and that the (jhange in the magni- 

 tude of the radiating surface is neglected. He has calculated the specific 

 heat of water by the new formula now derived, and finds that the equa- 

 tion fc=l-f0.000425f represents this constant at IP. {Wied. Ann., 1880, 

 II, X, 284.) 



Pfaundler has published a criticism on this paper, in which he points 

 out certain errors of experiment and assumption. The process of Reg- 

 nault, as modified by Berthelot, is the most exact known, and is free from 

 aU objection. In this the variation of temperature during the cooling 

 of the body is measured at regular intervals; then the calorimeter con- 

 taining the substance is brought back to the initial temperature and 

 made to pass through all the temperatures observed in the first experi- 

 ment, and the loss of heat is measured. ( Wied. Ann., 1880, II, xi, 237 ; 

 J. Phys., January, 1881, x, 43, 47. See also Berthelot, J. Phys., Febru- 

 ary, 1881, X, 79.) 



Latschinoft" has modified the lecture experiment proposed by Tyndali 

 for showing the inequality of the specific heats of solids. Since the den- 

 sities of the materials are not the same, the surfaces of various spheres 

 of the same weight are not equal, and an error is introduced. This the 

 author obviates by employing hollow spheres of the same weight and 

 the same diameter. In place of a plate of wax, the author places the 

 heated spheres on the surface of a transparent jelly of gelatin, and notes 

 their unequal penetration. (J. Pkys.-Chrm. Soc. Eusse, xii, 131 ; J. Phys., 

 September, 1881, x, 418.) 



LIGHT. 



1. Production and velocity. 



Michelson has made a research to test the truth of Fresnel's theory, 

 that the ether which is inclosed in optical media partakes of the motion 



