8 PRESSURE OF GASES 17 



the case with all the gases in the table of 7 ', except 

 hydrogen. 



Probably both influences occur in nature, and the 

 numbers inKegnault's table seem to show that in the 

 case of most gases the influence of cohesion is predominant 

 so long as the pressure lies within certain limits. But 

 when higher pressures are employed all gases exhibit, 

 according to the observations ofNatterer, 1 Amagat, 2 and 

 Cailletet, 3 the same behaviour as under lower pressure is 

 noted with hydrogen. The product PV increases with the 

 pressure P, because on account of the dimensions of the 

 molecules the volume V cannot diminish so much as the 

 law requires. 



These considerations, which we shall repeatedly have 

 again to take up and extend, 4 show that the departures from 

 the strict law can also be explained by the theory. Since 

 the probability of the theory is, therefore, in no respect 

 prejudiced by the inexactness of Boyle's law, we are 

 entitled to draw further inferences and conclusions, first of 

 all, from the simple theory, and to reserve their correction 

 for later chapters. 



9. Increase of Pressure by Heat 



Bernoulli also saw that his theory accounted not only 

 f or B o y 1 e ' s law, but also for the observed increase in the 

 pressure of a gas to which heat is communicated. Ac-' 

 cording to the laws of thermodynamics heat is energy ; an 

 increase of heat, therefore, consists in an augmentation of 

 the speed of the molecular motion, and this increase of 

 speed entails increase both in the number of impacts of 

 the molecules of the gas against the vessel and also in 

 the strength of these impacts. For a double reason, 

 therefore, the resultant action of the impacts in a given 



1 Pogg. Ann. Ixii. 1844, p. 132, xciv. 1855, p. 436. 



2 Comptes Rendus, Ixxxviii. 1879, p. 336 ; Ann. Chim. [5] xix. 1880, p. 345, 

 [6] xxix. 1893, p. 68. 



3 C. R. Ixx. 1870, p. 1131, Ixxxviii. 1879, p. 61 ; Journ. de Phys. viii. 1879, 

 p. 267. 



4 See Chap. IV. 40-51. 



C 



