76 Intelligence and Miscellaneous Articles. 



the smallness of the conducting-power of gases, the above objection 

 is not refuted. For the case in which the consideration of the regular 

 arrangement of the molecules is taken as basis, it is plain that the excess 

 of temperature must travel from one layer to another with a velocity 

 which cannot be different from that of sound. And the same follows 

 also from the investigation of the irregular motions. 



But even if heat is propagated in gases with the same velocity as 

 sound, the above objection is still not justified. It will only be so 

 when it necessarily follows from the new theory of gases, that an 

 excess of temperature in a given place of the gas is not only very 

 rapidly, but also completely and without loss imparted to the neigh- 

 bourhood. On the other hand, the objection is refuted if it can be 

 shown that the influence of an increase of temperature extends indeed 

 in a short time to great distances, but that the force of this in- 

 fluence is continually weaker with an increase of distance, and for a 

 short time can be infinitely small, even for small distances. 



Theory leads to this result when it is remembered that the mo- 

 tions of gas molecules take place in all possible directions, and, what 

 is essential, that this irregularity has its reason in the oblique impacts 

 on each other of the molecules. For on oblique impact one molecule 

 does not interchange its velocity with another ; the entire excess of 

 one molecule is not transferred to another, but in the mean only one- 

 half, assuming that the directions of the motions of the molecules 

 towards the line which joins their centres at the moment of impact 

 have all possible inclinations. In this consists the special peculiarity 

 of the conduction of heat, the tendency towards an equalization of 

 temperature. Hence of the downward moving molecules of the 

 upper layer, the entire excess of temperature in them will not be 

 transferred to the lower one, but only one-half ; that is £ of the en- 

 tire excess of the layer : from this to the next there will only be ^ 

 of the original excess of the first layer, and so on. If even the excess 

 of temperature after -g-^-g- of a second prevails to the distance of a 

 metre, this influence must be infinitely small, if we consider how 

 great is the number of layers into which, according to Maxwell's 

 calculation of the mean path of a molecule, a gas layer a metre in 

 thickness must be divided. Hence a considerable time must elapse 

 before the vis viva which has traversed a metre amounts to so much 

 as to be perceptible. 



The following conclusion arises from what has been said. The 

 velocity of the propagation of sound and of heat, that is to say con- 

 duction, are equal. The propagation of sound is distinguished from 

 that of heat by the fact that in the first case the difference of vis 

 viva produced anywhere by contraction or expansion is imparted 

 without loss to the adjacent mass, while this is not the case in the 

 propagation of heat, with the excess of vis viva produced by increase of 

 temperature, inasmuch as only one-half of this is imparted to the 

 neighbourhood. 



The assertion that heat has as great a velocity of propagation as 

 sound, appears contradictory to experiment. This is not the case. 

 Let us consider, for example, the following simple case of propagation. 



