82 Prof. Clausius on Molecular Motion, 



Ballot, inasmuch as it affords me a desired opportunity of com- 

 pleting this part of my theory (which was perhaps discussed too 

 briefly in my paper), and to prevent thereby further misunder- 

 standings. 



(2.) It is assumed in the objections, that the molecules traverse 

 considerable spaces in straight lines ; this appears prominently 

 in the second objection, in which it is said that a molecule must 

 have had to traverse the room many times in a second. This 

 assumption can, however, in nowise be considered as a necessary 

 consequence of the views advanced by me concerning the condi- 

 tions of gases. Amongst the conditions which must be satisfied if 

 Mariotte and Gay-Lussac^s law for a gas is true with perfect 

 strictness, I have adduced the following, — "that those portions 

 of the path of a molecule throughout which the molecular forces 

 are of influence in sensibly altering the motion of the molecule, 

 either in direction or velocity, must be of vanishing value com- 

 pared with those portions of the path throughout which such 

 forces may be considered as inactive." Now in actually exist- 

 ing gases, Mariotte and Gay-Lussac^s law is not strictly, but 

 only approximately true ; and it hence follows that in them such 

 first portions of the paths of the molecules must be small, but 

 not vanishingly small, compared with the entire paths. Inas- 

 much, now, as one of the fundamental conditions upon which 

 the whole theory rests is that the molecular forces are only 

 eftective at small distances from the molecules, a path which 

 is very great in comparison to the sphere of action of a molecule 

 may yet, considered absolutely, be very small. 



By a few simple considerations, an approximate idea may be 

 formed of the mean magnitude of the paths traversed by the 

 separate molecules : I purpose endeavouring to elucidate this in 

 what jjnmediately follows. 



(3.) For this purpose it will be advisable to prefix some re- 

 marks concerning the manner in which it is possible to view 

 the molecular forces, and what has accordingly to be understood 

 by the sphere of action. These remarks are not to be considered 

 as an essential part of the subsequent development, but are 

 merely intended to fix our ideas. 



If we do not take into account the forces of chemical affinity, 

 and only consider such molecules as are chemically indifferent to 

 one another, I imagine that there are still two forces which 

 are to be distinguished. I believe, namely, that when two 

 molecules approach one another an attraction is at first exerted, 

 which begins to be of sensible effect even at some distance, and 

 which increases as the distance diminishes ; but that, wlien the 

 molecules have arrived into the immediate neighbourhood of one 

 another, a force comes into play which seeks to drive them 



