19 PRESSURE OF GASES 37 



to a diminution of temperature, and vice versa. The change 

 of temperature which occurs is therefore 



cp V 



Here c denotes the specific heat at constant volume and 

 not that at constant pressure. That this is so we shall 

 easily see by again analysing the procedure. Without 

 transgressing the law of the conservation of energy we can 

 thus picture the transaction ; that the work of compression 

 p SF first produces a progressive velocity a of the gas, the 

 volume being diminished without the to-and-fro motions of 

 the molecules being altered, and that then, on the piston 

 ceasing to move forwards, the energy of the progressive 

 motion communicated to the gas is transformed into heat 

 without change of volume by the collisions of the molecules. 

 In the calculation, therefore, it is the specific heat at constant 

 volume that is to be taken into account. 



20. Vaporisation 



Many gaseous bodies are condensable into liquids by 

 application of pressure only, without the necessity of re- 

 moving heat from. them. Such substances are called 

 vapours, in contradistinction to gases proper. 



The cause of liquefaction by pressure alone we can only 

 look for in the forces of cohesion. If the molecules of a gas 

 are brought nearer each other by increase of pressure, those 

 forces are exerted in greater degree ; and it may happen 

 that, if the molecules are brought near enough together, 

 their action is so much increased that the molecules cannot 

 separate anymore from each other. For this it is necessary 

 that the kinetic energy of the molecules shall be no longer 

 sufficient to overcome the energy of the cohesive forces. If 

 this limit is reached, the vapour begins to change into 

 liquid. 



There is now a condition of equilibrium, in which one 

 part of the substance remains liquid and another hovers 

 above the liquid as vapour. In this vapour the molecules 



