Mr. W. J. M. Rauklne on the Mechanical Action of Heat. 11 



Real specific heat is consequently expressed by the following 

 equations : — 



For one atom : — 



dq_ SkM 

 dr 2Cn/M 

 For unity of weight : — 

 dQ,_ 3k 

 dr' 



'2Cnfj, 



For so much of a perfect gas as occupies 

 unity of volume under unity of pressure at 

 the temperature of melting ice : — 

 dq_3kM. 

 V~2C/i 



(XIII.) 



The laws estabhshed experimentally by Dulong, that the spe- 

 cific heats of simple atoms, and of certain groups of compound 

 atoms, bear to each other simple ratios, generally that of equality, 

 and that the specific heats of equal volumes of all simple gases 



are equal, show that the specific factor depends on the che- 

 mical constitution of the atom, and thus confirm the conjectm-e 

 I have stated I'especting the coefficient k. 



As 1 shall have occasion, in the investigation which follows, 

 to refer to and to use the equation for the elasticity of vapours 

 in contact with their liquids, which I published in the Edinburgh 

 New Philosophical Journal for July 1849, I shall here state ge- 

 nerally the nature of the reasoning from which it was deduced. 



The equilibrium of a vapour in contact vdth its liquid depends 

 on three conditions : — 



First. The total elasticity of the substance in the two states 

 must be the same. 



Secondhj. The superficial-atomic elasticity must vary conti- 

 nuously ; so that if at the surface which reflects light there is an 

 abrupt change of density (which seems almost certain), there 

 must there be two densities corresponding to the same super- 

 ficial-atomic elasticity. 



Thirdly. The two forces, which act on each stratum of vapour 

 parallel to the surface of the liquid, namely, the ])reponderance 

 of molecular attraction towards the liquid, and the difierence of 

 the superficial-atomic elasticities at the two sides of the stratum, 

 must be in equilibrio. 



Close to the surface of the liquid, therefore, the vapour is 

 highly condensed. The density diminishes rapidly as the di- 

 stance from the hquid increases, and at all appreciable distances 



