336 Mr. A. M. Worthington on. the 



case, its disadvantages as well as advantages. A multitude 

 of phenomena of the classes mentioned receive an easy expla- 

 nation ; but the explanation of others has become more trouble- 

 some than before. The mere mathematical difficulty of dealing 

 with a number of molecules in motion renders it impossible, 

 on the supposition of molecular motion, to frame a satisfactory 

 theory of internal equilibrium in solids or liquids. We are 

 obliged therefore either to invent new methods, or to return to 

 the old point of view; and it is important therefore, while 

 the more energetic minds are inventing new statistical methods 

 of dealing with molecules and substituting the new conception 

 of a transfer of momentum for the old one of a "force/'' to 

 examine the statical theory to ascertain whether a perfectly 

 philosophical use may not be made of it. For it often happens, 

 in the observational sciences, that the hypotheses of one theory 

 are completely represented in corresponding hypotheses of 

 another theory, and that the results of the one can be trans- 

 lated into the language of the other ; so that an impartial 

 observer will regard the hypotheses of either rather as con- 

 ventions between which there may indeed be some choice on 

 the ground of convenience and suitability, than as statements 

 of fact whose truth or falsehood is at issue. Now, it is easy 

 to, see that the relation between the statical and dynamical 

 molecular hypotheses is one of this kind, and that to the pro- 

 perties which we assign to the molecules on the one theory 

 there are corresponding properties assigned on the other. 



Thus, in either, the molecules are regarded as attracting each 

 other with a force which is a function of the distance only, 

 independent of the temperature and vanishing when the dis- 

 tance becomes sensible. On the statical theory the molecules 

 are supposed to be kept apart by a repulsive force which varies 

 with the temperature, and which is also a function of the 

 distance, but which turns out to be exerted only between 

 adjacent layers of molecules. On the dynamical theory the 

 equivalent property is the motion itself, combined in some 

 forms of the theory with an elastic repulsion between mole- 

 cules in collision, causing them to separate again (various 

 subordinate hypotheses having been suggested to account for 

 this latter property). And the explanation of the difference in 

 molecular structure between fluids and solids offered on the 

 one theory also corresponds closely with that belonging to the 

 other. 



And, again, it is significant to observe that, so far as the 

 internal equilibrium of substances is concerned, the concep- 

 tion of molecular motion stands in the same relation to the 

 statical conception as the modern theory of a dielectric does 



