20 PRESSURE OF GASES 39 



which is greater than that which on the average falls to 

 these molecules, and thus for those that are left behind 

 there remains a less amount of energy than they possess 

 hitherto on the average. It is thus explained on our theory 

 that a liquid cools by vaporisation. The heat that has dis- 

 appeared has gone in doing the work of expansion. 



21. Absorption and Adsorption 



Quite the same state of things occurs when a gas or 

 vapour is dissolved in a liquid which is not of the same sub- 

 stance as occurs in the phenomenon of absorption. In this 

 case the gaseous substance throngs into the liquid in conse- 

 quence of the motion of its molecules, and is held fast by 

 the attraction exerted on it by the liquid ; and this process 

 goes on until equilibrium between evaporation and conden- 

 sation occurs. 



On the absorption of a gas by a liquid, heat is developed 

 which is greater in amount than the latent heat of vaporisa- 

 tion. As the latter is equal to the sum of the kinetic energy 

 of the molecules of the gas and the potential energy required 

 to overcome the cohesion in the liquefied gas, it follows 

 that still more energy than this is required to separate the 

 gas from the liquid which has absorbed it. Hence there is 

 no doubt as to the existence of an attraction of the gas by 

 the liquid. 



If the force with which the liquid, when in the state of a 

 saturated solution, retains the gas were as great as if the 

 liquid were pure, the number of the molecules of gas 

 absorbed by the liquid would be exactly proportional to the 

 mmiber of molecules remaining above it in the gaseous 

 /state, and Henry's law, that the mass of gas absorbed 

 increases proportionally to the acting pressure, would hold 

 strictly. But as this supposition cannot be accurate when 

 large masses are absorbed, Henry's law can only approxi- 

 mately represent the truth. 



A rise of temperature also increases the energy of the 

 molecules in the absorbed gas ; they will, therefore, at a 

 higher temperature come oftener into a position of being 



