96 MOLECULAR MOTION AND ITS ENERGY 41 



As regards the coefficient of expansion, it follows from 

 the reasons given that it is not at all constant under all 

 circumstances, but that it depends on temperature and 

 pressure in such wise that with rising temperature or falling 

 pressure it decreases, and that at a sufficiently high tem- 

 perature or a sufficiently low pressure the expansion- 

 coefficient for every vapour attains the same value as that of 

 atmospheric air. 



42. Saturated Vapours. Absolute Boiling-point 



A problem to which the endeavours of experimentalists 

 in this direction have been especially directed consists in 

 the measurement of the highest pressure which a vapour can 

 attain in dependence on the temperature. The theoretical 

 investigators have also occupied themselves very greatly 

 with the condition of vapours at their maximum pressure, 

 or of saturated vapours, as they are called. Interesting 

 as are these researches, and important as their results may 

 be in themselves, this limiting case has less significance for 

 the theory of gases than for a corresponding theory of liquids. 



For the state of equilibrium of a saturated vapour which 

 lies above its liquid is characterised by this, that the 

 equilibrium is maintained by vaporisation and condensation 

 at the surface of the liquid. The molecules of the vapour 

 which in their to-and-fro motion strike the liquid surface 

 will not all bounce back, but a part will be retained by the 

 force of cohesion. On the contrary, it will happen just as 

 often that a particle of fluid which possesses sufficient speed 

 tears itself loose from its neighbours, and passes into the 

 vapour above. From this it follows that, in the state of 

 equilibrium of a saturated vapour, the kinetic energy of the 

 vapour is equal to the work which is done by the forces of 

 cohesion during condensation to the liquid state. The 

 measurement of the maximum pressure of the vapour has 

 therefore this significance, that it gives a measure for the 

 energy-value of cohesion in the liquid state, while for the 

 determination of the properties and laws of the vapour 



