ON THE KINETIC THEORY OF GASES. 215 



condition (2) that our molecules are not interfered with by 

 any body not a member of their own family. But the 

 luminiferous ether is such a body, and therefore, if it have 

 the properties attributed to it of imparting energy to or 

 receiving it from gas molecules, it is itself a part of the 

 material system under discussion, and our conclusions can- 

 not be safe unless we take it into account. 



22. It is worth while to note also that no actual gas 

 under experiment can accurately satisfy the condition (2) 

 required by the analysis. For collisions must occur, not 

 only between the molecules of the gas, but between them 

 and the walls of the containing vessel, and in the latter 

 case the condition (2) cannot be satisfied. 



23. The solution of this difficulty, when it is discovered, 

 will probably be found intimately connected with the solu- 

 tion of another question in the kinetic theory, namely, 

 What is the relation between kinetic energy and temperature ? 

 This question has been discussed by Tait, to whose reasoning 

 I shall refer later. 



24. On the deviation of gases from the laws of Boyle 

 and Charles. 



When a gas undergoes compression at constant tempera- 

 ture, it is found that the pressure increases as the volume 

 diminishes less rapidly than it would do were the relations 

 accurately expressed by the equation^ = C/. The devia- 

 tion becomes more marked as the compression increases, 

 and finally a portion of the compressed gas becomes liquid. 

 So that a state is reached in which if any more gas at the 

 same temperature were forced into the same volume, the 

 gas so forced in would all take the liquid form, and the 

 pressure of the still gaseous portion of the substance would 

 be unaltered. The degree of condensation required to pro- 

 duce this result is greater at higher than at lower tempera- 

 tures ; and at a certain temperature, called the critical 

 temperature for the substance in question, no degree of con- 

 densation converts any of it into liquid, or else the liquid 

 form of the substance is indistinguishable from its vapour. 

 The actual relations between/, v and t for certain gases 

 within certain limits have been determined with great 

 accuracy by the researches of Andrews, and more recently 



