48 IDEAL AND ACTUAL GASES 111 



molecules of the same kind, there would be always some of 

 greater mass. The number of the latter would depend on 

 the frequency of the favourable cases of collision, and there- 

 fore, also, chiefly on the number of collisions that occur, so 

 that in a denser gas wherein the molecules collide more 

 frequently there will be also more molecules of greater mass. 



49. Play fair and Wanklyn's Explanation of the 



Anomalies 



The hypothesis last expounded forms the basis of the 

 explanation of the anomalies regarding vapour-densities that 

 has been given by Playfair and Wanklyn, 1 an explana- 

 tion which embraces all the other deviations of actual gases 

 from the theoretical laws. 



According to what has already been said regarding the 

 hypothesis, it is only necessary to remember the mechanical 

 definition of temperature, given before in 14, to see at once 

 the possibility of the explanation of all anomalies. Accord- 

 ing to 29, the mean value of the kinetic energy of a 

 molecule, even when of different kinds, forms the measure 

 of temperature. Consequently, in a gas whose molecules 

 are either wholly or in part bound together to form larger 

 aggregates, the kinetic energy contained in unit volume is 

 less than before the aggregation ; or, more simply expressed, 

 the pressure is lowered by the aggregation of molecules if 

 the rise of temperature resulting therefrom is compensated. 

 Hence a gas whose molecules may combine together may 

 be more easily and strongly compressed than an ideal gas 

 whose molecules are unalterable massive points. A deviation 

 from Boyle's law will therefore arise in the direction 

 shown by most of the gases included in the table of 7, with 

 the single exception of hydrogen, the behaviour of which 

 has been already described elsewhere ; and determinations 

 of vapour-density will therefore give higher values the 

 higher the pressure during the measurement, as was found, 

 for instance, by Alex. Naumann to be the case for acetic 

 acid. 



1 Trans. Roy. Soc. Edin. xxii. pt. 3, 1861, p. 441 ; Ann. Cliem. Pharm. 

 cxxii. 1862, p. 247. 



