256 Mr. F. T>. Brown on Molecular Attraction. 



bining-power, that the power of attraction residing in the 

 oxygen atoms is all concentrated upon the hydrogen atoms, just 

 as we might conceive all the attractive power of the earth 

 concentrated on the moon, thus leaving all other bodies in its 

 neighbourhood free from the influence of gravity. We thus 

 invest matter with two separate forms of attraction, differ- 

 ing entirely in their mode of action, and having indeed nothing 

 in common. 



Although there is no inconsistency in this, yet it would 

 seem to be simpler to assimilate to a certain extent chemical 

 attraction and gravitation. Let us suppose, then, that the act 

 of chemical combination in no wise alters the power of attrac- 

 tion which the combining atoms exert upon surrounding 

 bodies, and let us see what effect this hypothesis has upon the 

 explanation of various phenomena. 



That this assumption is not purely arbitrary is proved by 

 the mere occurrence of any and every chemical reaction ; for 

 it is evident that no such reaction could take place unless the 

 force of chemical affinity were exerted between atoms which 

 are still held in combination in different molecules. Take, 

 for example, any simple reaction, such as the neutralization of 

 caustic soda by hydrochloric acid: when the molecules of sodium 

 hydrate are brought into proximity with those of the acid, 

 some of the atoms are interchanged, and molecules of water 

 and of sodium chloride result. Now this can only take place 

 in two ways: in the one we may imagine a constant sponta- 

 neous decomposition and recomposition of the several mole- 

 cules to take place, so that there are in the alkaline liquid a 

 considerable number of free atoms ready to act on other free 

 atoms in the acid liquid ; in the other we must allow that the 

 force of affinity inherent in the chlorine atoms stretches 

 across, as it were, to the sodium atoms in the sodium hydrate. 

 Of the constant decomposition and recomposition necessitated 

 by the first process of reaction, we have, at any rate in the 

 majority of cases, no evidence whatever; on the contrary, the 

 agreement of the observed vapour-densities of substances with 

 those required by Avogadro's law renders such molecular in- 

 stability extremely improbable; we are therefore driven to 

 allow that affinity does act from molecule to molecule. 



The hypothesis above indicated enables us to explain to a 

 certain extent the relative volatility of substances, and goes 

 far to render a satisfactory account of molecular compounds. 

 We all, I presume, look upon the maximum vapour-tension 

 of a substance at a given temperature as affording, to a certain 

 extent, a means of estimating the attraction which its mole- 

 cules exert among themselves : if there is considerable attrac- 



