278 PHENOMENA DEPENDENT ON MOLECULAR PATHS 103 



At the time when Clausius laid the foundation of the 

 kinetic theory the low conductivity of gases and vapours 

 was adduced, as has been already mentioned in 61 at the 

 beginning of Part II., as the most weighty objection to the 

 correctness of the new hypothesis. It was asserted that in a 

 medium the molecules of which are moving with great speed 

 the heat, which consists in the energy of motion of the 

 molecules, must also be transferred with similar speed from 

 one spot to another. This objection we may still attempt 

 to maintain, even if the similar objection which has been 

 raised in respect to the diffusion of gases is already removed. 

 For although it has been shown that on this theory the 

 mixing of two gases does not proceed with the speed at which 

 the molecules move, but very slowly, because the molecules 

 frequently collide and therefore traverse only very short 

 distances in spite of their large speed, yet this answer to 

 the doubts derived from the phenomenon of diffusion cannot 

 without more consideration be applied to the phenomenon of 

 conduction, since in diffusion there is a question of a trans- 

 ference of mass, and in conduction, on the contrary, of a 

 transference of energy. 



This difference is best illustrated by an example which 

 seems to lead to a conclusion by analogy. A very common 

 piece of physical apparatus consists of a row of elastic balls 

 which hang in contact and are used for experiments on the 

 collision of balls. If, for instance, we allow one of these 

 balls to fall against the remainder of the row, it comes to 

 rest itself on the collision if all the balls are of the same 

 size, and all the others remain at rest except the last, which 

 flies off from the rest with the same speed with which the 

 first struck the row. In this experiment the energy alone 

 of the motion has been propagated, and, indeed, with very 

 great speed, without the mass through which the trans- 

 mission was effected that is, the row of intermediate balls 

 being carried on with the energy. We might likewise 

 conclude that the energy of the heat-motion may be very 

 rapidly transmitted by means of a group of molecules, which 

 now and then collide and mutually transmit energy to each 

 other in the collisions, without the molecules which effect 



