42 IDEAL AND ACTUAL GASES 97 



state the observation of unsaturated or superheated vapours 

 is of greater importance. 



We shall, therefore, here touch upon only one point of 

 especial interest which has resulted from the observations of 

 Mendelejeff l and Andrews. 2 As the observations of 

 Frankenheim and others on the capillary rise of liquids 

 in narrow tubes at different temperatures have shown, the 

 cohesion of a liquid decreases greatly as the temperature 

 rises, whence we should conclude that heat probably pro- 

 duces molecular motions in liquids, just as in gases, by 

 which their cohesion is diminished. If we increase the 

 temperature and this molecular motion more and more by 

 addition of heat, we may imagine a point reached at which 

 the forces of cohesion cease to act and the capillary constant 

 which measures them is zero. 



On the other hand, the vaporisation which results from the 

 rise of temperature and the consequent increase of molecular 

 motion is prevented, or at least hindered by pressure. We 

 may imagine the process to be this : that pressure and dimi- 

 nution of volume bring the molecules of the vapour nearer 

 together, and cause their cohesive forces, which increase with 

 diminishing distance between the molecules, to come more 

 strongly into play, so as to overcome the expansive force due 

 to the motion of the molecules. This is, however, pos- 

 sible only so long as the kinetic energy of the molecular 

 motion is not too great ; if by the addition of heat this 

 energy should become so great as to exceed the sum total 

 of the potential energy of the forces of cohesion which is 

 lost by two molecules which, from being widely separated, 

 come into contact with each other, it is no longer possible to 

 cause the molecules to join together, and it is, therefore, 

 also no longer possible for the vapour to be changed into 

 liquid by pressure. The temperature necessary for this, 

 which Mendelejeff calls the absolute boiling-point, may, 

 according to our former remarks, be determined also from 



1 Ann. Chem. Pharm. cxix. 1861, p. 1 ; Pogg. Ann. cxli. 1870, p. 618. 



2 Report Brit. Ass. 1861, ii. p. 76 ; Ann. Chem. Pharm. cxxiii. 1861, p. 270 ; 

 Phil. Trans, clix. pt. 2, 1869, p. 575; Pogg. Ann. Erg.-Bd. v. 1871, p. 64; Proc. 

 Roy. Soc. xxiii. 1875, p. 514. 



H 



