12 MOLECULAR MOTION AND ITS ENERGY 6 



CHAPTEK II 

 PRESSURE OF GASES 



6. Boyle's Law 



THE hypothesis, which we have described, of the to-and-fro 

 motion of the molecules of a gas in straight paths, of their 

 collisions and subsequent separations, and of their striking 

 against the walls of the containing vessel, furnishes a very 

 simple explanation of the cause of the pressure which the 

 gas exerts. This pressure results from the series of impacts 

 of the molecules, as they move to and fro, against the 

 enclosure. As the first test of the admissibility of our 

 hypothesis, we have to see whether this explanation of the 

 pressure is in agreement or not with the laws of gaseous 

 pressure that have been deduced from experiment. 



The law with which we have first to deal is in Germany 

 generally called Mariotte's law, because Mario tte 

 enunciated it at the head of his essay ' De la Nature de 

 FAir/ which first appeared in 1679. As, however, there can 

 be no doubt that the discoverer 1 of this law is Robert 

 Boyle, 2 who determined it seventeen years earlier, I shall 

 follow the English custom of calling it Boyle's law. 



That this law viz. that the density and pressure of a 

 gas are proportional to each other is not in contradiction 

 with the kinetic theory, but, on the contrary, is a necessary 

 result of the hypothesis of rectilinear motion, was proved 

 by Daniel Bernoulli, 3 the originator of this hypothesis. 



1 See, for instance, Muncke in Gehler's PhysiJcal. Worterbuch, 1828, 

 iv. pp. 1026, 1028. 



2 ' A Defence of the Doctrine touching the Spring and Weight of the Air,' 

 London 1662, Pt. II. Chap. V. 



3 Hydrodynamica, Argentorati 1738, Sect. X. p. 200 ; reprinted in German, 

 Pogg. Ann. cvii. 1859, p. 490. 



