THEOE Y OF A PPROXIMA TION. 9 1 



gases nearly obey the law of Boyle and Marriotte ; that 

 they nearly expand, by heat at the uniform rate of one 

 part in 272*9 of their volume at o for each degree centi- 

 grade ; and that they more nearly fulfil these conditions 

 the more distant the point of temperature at which we 

 examine them from the liquefying point, we pass by the 

 principle of continuity to the conception of a perfect gas. 

 Such a gas would probably consist of atoms of matter at 

 so great a distance from each other as to exert no attrac- 

 tive forces upon each other ; but for this condition to be 

 exactly fulfilled the distances must be infinite, so that an 

 absolutely perfect gas cannot exist. But the perfect gas 

 is not merely a limit to which we may approach, it is a 

 limit passed by at least one real gas. ' It has been shown 

 by Despretz, Pouillet, Dulong, Arago, and finally Regnault, 

 that all gases diverge from the Boylean law, and in nearly 

 all cases the density of the gas increases in a somewhat 

 greater ratio than the pressure, indicating a tendency on the 

 part of the molecules to approximate of their own accord, 

 and condense into liquid. In the more condensible gases 

 such as sulphurous acid, ammonia, and cyanogen, this 

 tendency is strongly apparent near the liquefying point. 

 Hydrogen on the contrary diverges from the law of a 

 perfect gas in the opposite direction, that is, the density 

 increases less than in the ratio of the pressure \ This is a 

 singular exception, the bearing of which I am unable to 

 comprehend. 



All gases diverge again from the law of uniform ex- 

 pansion by heat, but the divergence is less as the gas in 

 question is less condensible, or examined at a temperature 

 more removed from its liquefying point. Thus the perfect 

 gas in this respect must have an infinitely high tempera- 

 ture. According to Dalton's law each gas in a mixture re- 

 tains its own properties wholly unaffected by the presence 

 * Jamin, ' Cours de Physique,' vol. i. pp. 283-288. 



