On the Thermodynamics of Volatile Liquids. 477 



endosmose " * the current may also produce work, when the 

 pressures are different on the two sides of the diaphragm. 

 It is easy to prove that the electromotive force e which cor- 

 responds to this case is that of the " diaphragm-currents " 

 discovered in 1859 by Quincke f. 

 Moscow, April 1876. 



LVI. Application of the Mechanical Theory of Heat to the 

 Study of Volatile Liquids: Simple Relations between the 

 Latent Heats, Atomic Weights, and Tensions of Vapours. 

 By Raoul PictetJ. 



[Plate IV.] 



IN almost all recent works on the mechanical theory of heat 

 the authors devote the first chapters to the general equa- 

 tions of the thermic problems — that is to say, to the principle 

 of equivalence, and to the second principle, that of the equality 

 of restoration in reversible cycles ; they then apply directly 

 the general equations found to the changes of state of bodies, 

 and to the phenomena of calorimetry connected with them. 

 Sady Carnot, Clapeyron, and afterwards Clausius, Dupre, 

 Zeuner, Hirn, &c.,have apparently followed analogous courses 

 and been conducted to very similar results. Nevertheless the 

 physical solution of the problems analytically investigated is 

 always obscure : it cannot get rid of the confused train of 

 equations which constantly veil a positive comprehension of 

 the results of the analysis. 



It is, in great measure, in consequence of an abuse of for- 

 mulae which are not translated into ordinary terms that trea- 

 tises on physics give no information upon the relations which 

 connect the various properties of volatile liquids. The aim 

 we propose to ourselves is, to give some laws representing 

 with sufficient exactness the notions at present held on this 

 question. 



First let us settle the principal factors of the physical 

 problem. 



We have a kilogramme of any liquid whatever, at the tem- 

 perature t°. At this temperature it is caused to pass from the 

 liquid into the gaseous state ; in doing this it absorbs a 

 quantity of heat represented by X calories — the latent heat of 



* These phenomena, discovered by Prof. Fr. Reuss, of Moscow, were 

 investigated chiefly by Wiedemann, Pogg. Ann. vol. lxxxvii. ; see also 

 Galvanismus, vol. i. p. 576. 



t Pogg. Ann. vols. cvii. & cxiii. Wiedemann, Galvanismus, vol. i. 

 p. 946. 



\ Translated from the Bibliotheque Tlniverselle, Archives des Sciences 

 Physiques et Naturelles, Jan. 15, 1876, vol. lv. pp. 66-83. 



