306 Dr. G. Gore on Changes of Temperature caused 



dQ ~dQ dv 

 dT_ ~di ~!yv 'It 



It" BQ ' 



dT 



dT dQ 



and the law that —r- has the sign of -j- again holds good. 



Consider, now, the meaning of the condition (IV.). Evi- 

 dently the meaning is that the potential of gravitation should 

 be a continuous explicit function of volume alone. To every 

 infinitesimal variation of volume corresponds an infinitesimal 

 variation of the potential. This condition is realized when 

 the body passes in a continuous manner from one configuration 

 of mechanically stable equilibrium to another. It is evident 

 that, when speaking of the contraction of a celestial body, we 

 mean a " grosso modo " continuous passing from one configura- 

 tion of stable equilibrium to another. Hence, resuming our 

 reasoning, we may say that the temperature of a celestial 

 body, uninfluenced by external forces and radiating heat, 

 must be sinking. But a rain of meteorites (as Lord Kelvin 

 has suggested) may bring quanta of energy sufficient to 

 compensate the loss of heat by radiation, because the sun and 

 a meteorite falling upon it constitute a mechanically unstable 

 system, and the condition (IV.) is not fulfilled. 



XXVII. Changes of Temperature caused by Contact of Liquids 

 with Powdered Silica Sfc. By Dr. G. Goke, F.jR.S* 



IT is already known that the physical properties of the 

 surface-layer of molecules of a solid or liquid substance 

 are in some respects different from those of the molecules 

 which constitute the inner portion of the mass, and that those 

 of a very thin film of liquid are not exactly like those of a 

 large bulk, It is further known that solid substances have 

 the power of condensing extremely thin films of liquids, 

 vapours, and atmospheric air upon their surfaces and in their 

 pores, and that the physical properties of the liquids and 

 vapours are thereby modified. 



According to Rticker, the specific electric conductivity of 

 a film of soap-solution increases as the thickness decreases 

 (< Nature,' June 1 and October 26, 1893, pp. 116, 625). The 

 superficial film of a liquid is usually very much more viscous 

 than the interior (Daniell, ' Text-book of Physics/ p. 258). 

 According to Overbeck, distilled water offers more, and 

 alcohol less, resistance to the vibrations of a pendulum at its 

 surface than in the interior of its mass (' Nature,' Feb. 24, 

 * Communicated by the Author. 



