14 MOLECULAR MOTION AND ITS ENERGY 7 



no proof of the exclusive claims of our kinetic hypothesis ; 

 but it allows us to judge for what substances, and under 

 what circumstances, the theory may be considered ad- 

 missible. 



Boyle's law is not obeyed by all substances in the 

 gaseous state. The vapours of liquid bodies do not obey it 

 except within certain limits of pressure and temperature, 

 and then only with moderate approximation. Even the 

 so-called permanent gases do not satisfy the law rigorously 

 and under all circumstances. 



,.,--' This was known to Boyle himself, and the inexactness 

 ^f the law has been confirmed by Musschenbroek and 

 a whole series of physicists, ancient and modern, such as 

 Despretz, Arago and Dulong, Pouillet, Kegnault, 

 Siljestrom, Mendelejeff and Kirpitscheff, Amagat, 

 Cailletet. 



It would be out of place here to enter fully into the 

 results of the numerous investigations undertaken to test 

 Boyle's law, as this work does not profess to be a complete 

 text-book of the physics of gases, and the more detailed text- 

 books l contain full information. A few examples are here 

 sufficient, which show how far the real gases depart from 

 Boyle's law under ordinary circumstances, that is, at mean 

 temperatures and under moderate pressures. 



If this law -were strictly exact, the product of the pres- 

 sure p into the volume v of a mass of gas would be a 

 magnitude which would not alter in value when the pressure 

 took the value P and the volume the corresponding value 

 F ; we should, therefore, have 



pv _-, 

 PV~ 



if, as we assume, the temperature did not alter. The 

 following table contains for a series of gases the values of 

 this ratio which Begnault 2 found on increasing the pres- 

 sure from_p = l atmosphere to P = 2 atmospheres. 



1 Winkelmann, Handbuch der Physik, i. p. 503 ; Ostwald, Allgemeine 

 Chemie, 2nd ed. i. p. 139 ; &c. 



2 M6m. de VAcad. de Paris, xxi. 1847, p. 329 ; xxvi. 1862, p. 260. 



