122 DIRECT PROPERTIES OF MOLECULES 341 



molecule of the substances investigated as not much less 

 than a millionth of a millimetre. We may, therefore, 

 consider ourselves justified in looking upon this measure 

 as a tolerably accurate limit of the size of the smallest 

 particles. 1 



123. 



The information given by the kinetic theory as to the 

 forces that act between atoms and molecules seems very 

 meagre in comparison with the widely extending knowledge 

 of their state which this theory affords. 



We know as a fact, directly proved by experiment, that 

 the forces exerted by gaseous molecules on each other when 

 separated by their average distances are of very small inten- v 

 sity. But this tells us nothing as to the same forces which 

 come into operation between two molecules when they 

 approach very near to each other. Our theory assumes that 

 forces come into play at a collision which drive the mole- 

 cules away from each other ; but, however much we have 

 discoursed of impacts and impulses in this book, we have 

 learnt nothing more as to the nature of these forces ; we 

 have remained in ignorance whether these forces are in- 

 stantaneous stresses, which come into play at the moment 

 of contact, or if they are of the kind generally assumed in 

 theories of capillarity, which act at only very small dis- 

 tances and rapidly fall off as the distance increases, while, 

 on the other hand, preventing a diminution of the distance 

 beyond a certain limit. 



If the theory of gases does not decide this question, it is 

 no ground for reproach of the theory ; but, on the contrary, 

 it is the source of a superiority which this theory has over 

 others. For the reason that it does not decide the question 

 which of the two hypotheses on the nature of molecular 

 forces, which seem possible and admissible, is the true one, 

 is that it is itself independent of the choice of either of 



1 [An admirable rts'ttmt of the experimental investigations on molecular 

 magnitudes is contained in K ticker's lecture 'On the Range of Molecular 

 Forces,' Journ. Chem. Soc. 1888, liii. p. 222. TB.] 



