Law of Molecular Force. 187 



shows the amount of experimental error possible in these very 

 difficult experiments, and also shows the degree of accuracy 

 which is to be looked for in the experiments on air and C0 2 . 



The only other experiments that I know to be on record 

 on cooling by expansion are those which Hirn made with 

 steam {TMorie Mecanique de la Chaleur) ; but before they 

 can be studied in the same manner as those on C0 2 and air 

 have been in this paper, an equation on the pure " Virial " 

 model will have to be constructed for steam. 



The fields of application for the molecular law of the inverse 

 fourth power are wide and inviting. In the theory of capil- 

 lary action it ought to lead to the accurate determination of 

 the size of the molecular domain in different substances under 

 specified conditions, or, in other words, of the number of mo- 

 lecules in unit mass of all bodies. In the theory of elasticity 

 its possibilities are vast ; for, applied in connexion with the 

 great accumulation of facts bearing on crystalline structure, 

 it ought ultimately to render possible a general theory of the 

 structure of molecules. But the most inviting direction for 

 immediate research is towards the law of variation of I with 

 variation in molecular constitution. It remains to be seen 

 whether the experimental data at present available are suffi- 

 cient to give a clue to it ; but there is no doubt that the pro- 

 gress of molecular physics and chemical dynamics requires 

 experimental data for the structure of the characteristic equa- 

 tions of a large number of bodies, for example of the members 

 of many homologous series of organic compounds ; for, before 

 we can hope to understand the atomic forces at play in chemical 

 action, we must possess a complete knowledge of molecular 

 actions. 



With ~2 + -4- to express the law of molic force through the 



whole range of distances from molecular up to astronomical, 

 one is tempted to speculate whether the law of the terms re- 

 presenting atomic or chemic force may not be expressed by 



one or more higher powers of -%, representing a force insen- 

 sible at molecular distances as the molecular term of molic 

 force is insensible at astronomical distances, but sensible at 

 atomic distances, with the associated idea that atomic distances 

 are exceedingly small compared to molecular. This concep- 

 tion would (speaking in a purely relative manner) reduce the 

 molecules almost to mathematical points,- and would almost 

 remove the difficulty as to the collisions of molecules. In 

 this manner we can endeavour to realize all the actions of 

 matter on matter as pure attractions. 



