4 FOUNDATIONS OF THE HYPOTHESIS 9 



parabola. This curvature of the path under the action of 

 /gravity will, however, be quite insignificant if the speed of 

 the molecules is very great. Since this condition is actually 

 fulfilled, as the numbers in 13 show, we may neglect this 

 curvature and consider the molecular motions in even heavy 

 gases as rectilinear. 



Of more importance is the fact that gases are not quite 

 free from cohesion of their particles, but exhibit distinct, 

 though very slight, traces of it. Two gaseous molecules, 

 however, can only attract each other when sufficiently near, 

 so that if a gas is not too strongly compressed, but is far 

 from the point of liquefaction, we are justified in represent- 

 ing an overpowering majority of its molecules as far enough 

 apart to be nearly always outside the range of their mutual 

 attraction ; and we may therefore assume that the small 

 amount of cohesion which does come into play is to be put 

 to the account of the rarely occurring cases when two mole- 

 cules now and then come very near each other. 



If we therefore represent the molecules of a gas as 

 moving in general in a straight line, and only changing 

 direction when two approach very closely, this view is 

 practically the same as that enunciated for the simple case 

 first given, the difference between them consisting only in 

 the substitution of a rapid, though gradual and continuous, 

 change of direction in the motion of two molecules on very 

 close approach to each other, in place, of a sudden rebound 

 on collision. 



The most essential point of our hypothesis is not thereby 

 ouched ; it remains true that a gaseous molecule moves 

 with uniform velocity in a straight line between every two 

 successive collisions with other molecules. 



t. 

 * 



5. Founders of the Kinetic Theory 



s When these views on the nature of the gaseous state 



i/were published in 1856 by Kronig, 1 and in 1857 by 



Clausius, 2 they aroused very special remark by their 



1 GrundzUge einer Theorie der Gase : first published separately in Berlin 

 by A. W. Hayn, and then in Pogg. Ann. xcix. 1856, p. 315. 



2 Ueber die Art der Bewegung, welche wir Wcirme nennen, Pogg. Ann. c. 



