61 MOLECULAR FREE PATHS 151 



conductors that it could not maintain a temperature in 

 which life could exist. 



62. Refutation of the Objections 



There would, indeed, have been no need of such a piling 

 up of objections to bring the conviction that in a theory 

 that has already obtained confirmation in so many points 

 there must be some conception liable to be misunderstood. 



The molecules of gas certainly move with that furious 

 speed, and also move in nearly straight paths, but only till 

 they strike some obstacle or collide with other particles. 

 This, however, occurs very often so often, indeed, that the 

 case in which a molecule of the atmosphere enveloping 

 us traverses a path of 400 metres without actual disturb- 

 ance hardly ever occurs ; but each air particle collides with 

 some other exceedingly often, indeed many million times a 

 second. 



This remarkable behaviour, which was brought to light 

 by a theoretical calculation carried out successfully by 

 Clausius 1 and was confirmed by Maxwell in a theoretical 2 

 and experimental 3 investigation closely connected with that 

 of Clausius, puts our theory in quite a different light. We 

 have not to consider the rectilinear backward and forward 

 motion of the molecules as a translatory motion, bound up 

 with enormous change of place and proceeding within wide 

 limits of space, but as consisting of a motion of molecules 

 among each other, proceeding tumultuously hither and 

 thither in straight zigzags and confined within a narrow 

 space ; the molecules thus execute such a motion that the 

 best representation of it is that of grains of corn shaken 

 about in a closed box. 



By this explanation of the character of the molecular 

 motion the objections that have been raised fall at once to 

 the ground ; for the supposition at the bottom of each, 

 viz. that a particle of air can in a second reach a place 



1 ' Ueber die mittlere Lange der Wege u.s.w.,' Pogg. Ann. 1858, cv. p. 239 ; 

 Collected Works, 2. Abth. 1867, p. 260 ; tfansl. Phil. Mag. [4] xvii. 1859, p. 81. 



2 Phil. Mag. 1860 [4] xix. p. 19 ; xx. p. 21. 



3 Phil. Trans. 1866, clvi. p. 249'. 



