with Remarks upon the Mechanical Thewy of Heat. 83 



asunder. For the view which it is here intended to take, it is 

 indifferent what kind of force this repulsive one is supposed to 

 be, that is, whether, as iu the case of sohd elastic bodies, it only 

 strives to separate the molecules when they are in actual contact 

 with a force equal to that with which they are pressed together, 

 or whether it is one which begins to act before the actual con- 

 tact of the molecules. In the same manner, we need not hei'e 

 discuss the question as to the source of these forces, whether 

 they are both to be ascribed to the particles of ponderable 

 matter themselves, or whether one of them is to be referred to 

 a more subtle substance, with which the ponderable particles of 

 matter are furnished. 



Let us now imagine two molecules moving in directions such 

 that, if they preserved them imchanged, they would not strike 

 one another, but pass by at some distance. Two cases may here 

 occur. If the distance is very small, the molecules which were 

 di-awn towards one another, even from some distance, by the 

 force of attraction, approach so closely that the repulsive force 

 comes into play, and a rebounding of the molecules results. If 

 the distance be somewhat greater, the paths of the molecules 

 only suffer a certain change of direction through the attractive 

 force, without the repulsive force being able to act. Finally, at 

 still greater distances, the effect of the molecules upon one 

 another may be altogether neglected. 



How great the distances must be in order that the one or 

 other case might occur, could not be determined universally, 

 even if we possessed exact knowledge of the molecular forces; 

 for the velocity of the molecules and the reciprocal inclination of 

 their paths are of influence. Nevertheless, mean values of these 

 distances may be obtained. We will therefore suppose that the 

 distance p is given for such a mean value, which forms the 

 boundary between the first and second case, and the meaning of 

 which we will define with greater precision in the following 

 manner : — If the centres of gravity of two molecules have such 

 directions of motion that if they were to proceed in those direc- 

 tions in straight lines they would pass by one another at a 

 distance greater than p, then the molecules only change their 

 courses to some extent through reciprocal attraction, without the 

 repulsive force coming into action between them. If, on the 

 other hand, this distance is less than p, the latter force also 

 comes into play, and a rebounding of the molecules takes 

 place. 



If, now, the latter case alone be considered as one of impact^ 

 and we do not concern om'selvcs with the changes of direction 

 which the force of attraction effects at greater distances, we may, 

 for what we have here to consider, rej)resent a sphere of radius 



G2 



