98 MOLECULAR MOTION AND ITS ENERGY 42 



the diminution of the capillarity with rise of temperature. 

 Mendelejeff has determined the height of this point for 

 several liquids. 



These theoretical views are in most excellent agreement 

 with the observations made by Andrews, and also earlier 

 by Cagniard de la Tour and Faraday. Andrews 

 called the temperature of the absolute boiling-point, the 

 critical point, which forms the limit between the vapour 

 state and that of a gas proper. At a temperature below this 

 critical point an elastic fluid can be condensed into a liquid 

 both by pressure and by cooling, that is, by either alone, 

 and thus deserves the name vapour. On the contrary, at a 

 temperature above the critical point there is no pressure 

 high enough of itself to make the fluid become liquid, and 

 both cooling and pressure must be applied together to pro- 

 duce condensation ; the fluid is then called a gas. 



Conformably to Mendele Jeff's theoretical interpreta- 

 tion, therefore, we must consider a gas to be a medium in 

 which the kinetic energy of the molecules is greater than 

 the sum total of the energy of the forces of cohesion which 

 may come into play on condensation to a liquid. In a 

 vapour, on the contrary, the energy of motion does not 

 reach this amount, but it is sufficient to overcome the part 

 of the cohesion-energy which from time to time comes into 

 play in the to-and-fro motions of the vapour molecules. 



A further investigation of these interesting limiting 

 states between gas, vapour, and liquid that have been 

 touched upon, and especially of the saturated state of 

 vapours, will some day or other * presumably form a bridge 

 by which a passage will be found from the kinetic theory 

 of gases to a kinetic theory of liquids a theory the ideas 

 underlying which have been already expressed by Clausius. 



For us, whose aim is the establishment of the laws of 

 gases, the foregoing suffices, on the one hand, to show 

 the necessity of an improvement of our theory, which has 

 otherwise approved itself in so many different respects, and, 



1 [Voigt has elaborated a kinetic theory of vaporisation and of liquids in 

 Gott. Nachr. 1896, p. 341 ; 1897, pp. 19, 261. See Phys. Soc. Abstracts, iii. 

 1897, p. 350 ; Science Abstracts, i. 1898, p. 545. TR.] 



