MODEIIX PHYSICS. 



ouch molcculo is the same for each gas, is due to 

 Waturston. They were contained in his paper of 

 184,0, and published by him in 1851. Both these 

 papers, however, appear to have been unnoticed by 

 all subsequent writers until 181)2. 



Meanwhile, in 1848, Joule's attention was called 

 by his experiments to the question, and he saw that 

 Herapath's result gave a means of calculating the 

 mean velocity of the molecules of a gas. For ac- 

 cording to the result given above, /> = J p r-; thus 

 tf = :j p/p t and y> and p being known, we tind v 2 . Thus 

 for hydrogen at freezing-point and atmospheric pres- 

 sure Joule obtains for v the value 6,055 feet per second, 

 or, roughly, six times the velocity of sound in air. 



Clausius was the next writer of importance on the 

 subject His first paper is in PoggendoHTs Annu- 

 len," vol. c., 1857, "On the Kind of Motion we call 

 Heat." It gives an exposition of the theory, and 

 establishes the fact that the kinetic energy of the 

 translutory motion of a molecule does not represent 

 the whole of the heat it contains. If we look upon 

 a molecule as a small solid we must consider the 

 energy it possesses in consequence of its rotation 

 about its centre of gravity, as well as the energy due 

 to the motion of translation of the whole. 



C'lausius' second paper appeared in 1859. In it 

 he considers the average length of the path of a 

 molecule during the interval between two collisions. 

 He determines this path in terms of the average 

 distance between the molecules and the distance 

 between the centres of two molecules at the time 

 when a collision is taking place. 



