152 PHENOMENA DEPENDENT ON MOLECULAR PATHS 62 



distant by more than 400 metres, is not at all made in the 

 kinetic theory. 



How in accordance with this we have to explain the 

 slowness with which the diffusion of gases proceeds is clear 

 and intelligible without further words. The reason for the 

 slowness of the conduction of heat is also easily seen if we 

 look more closely into the process in the way in which 

 Stefan 1 has first examined it. 



A heating of one region in a gas consists, according to 

 our theory, in an increase of the molecular speeds in this 

 region. The warmer particles therefore collide with greater 

 momentum against the colder ones near them, and thereby 

 impart a portion of that greater momentum, or, what is 

 the same thing, of their higher temperature, to their environ- 

 ment. In this transference of the energy of motion consists 

 the conduction of heat. This conduction would take place 

 with the speed of the molecular motion if at each collision 

 the striking particle so hit the one struck that the latter 

 moved on in the same direction as that in which the blow 

 occurred. But this happens only on the direct collision of 

 two molecules which were moving in the same or opposite 

 directions ; they then simply exchange their velocities, and 

 the whole excess of motion and heat is transferred from 

 the one particle to the other. Mostly, however, the particles 

 collide against each other obliquely ; then the particle 

 struck is thrust off in quite a different direction, and it 

 follows that it also receives a much less share of the excess 

 of energy of the other. The transference of heat is there- 

 fore not only impeded by its having to follow a zigzag 

 path hither and thither, instead of proceeding in a straight 

 line, but also by only a small fraction of any excess of 

 energy being in general imparted at each collision. It is 

 thus intelligible why a sensible heating cannot spread with 

 the speed of sound in the space occupied by a gas, but only 

 very slowly. 



The objections raised are not, therefore, hard to remove. 

 But the raising of them was of great importance for the 

 development of the kinetic theory, and was very beneficial 



1 Pogg. Ann. 1863, cxix. p. 492 ; Wiener Sitsungsber. xlvii. 



