CHEMICAL AFFINITY. 161 



bining particles be such that the correlative expansion ought 

 to be greater (if there were no chemical combination) than 

 that occupied by the total volume of the new compound, an 

 extra expanding power is evolved, and heat or expansion 

 ought to be produced in surrounding bodies. In other words, 

 if a a could be brought by physical attraction as near each 

 other as they are by chemical attraction brought near to b 5, 

 they would, from their increased proximity, produce an ex- 

 pansive power ultra the volume occupied by the actual chem- 

 ical compound A and B. The question, however, immedi- 

 ately occurs, why should the volume of the compound be lim- 

 ited and not occupy the full space equivalent to the expanding 

 power induced by the contraction or approximation of the 

 particles. As the distance of the particles is the resultant 

 of the contending contracting and expanding powers, this 

 result ought to express itself in terms of the actual volume 

 produced by the combination, which it certainly does not. 



Though I see some difficulties in Dr. Wood's theory, and 

 perhaps have not rightly conceived it, his views have to my 

 mind great interest, his mode of regarding natural phenomena 

 being analogous to that which I have in this Essay, and for 

 many years, advocated, viz. to divest physical science as 

 much as possible of hypothetic fluids, ethers, latent entities, 

 occult qualities, &c. My own notion of the heat produced 

 by chemical combination, though I scarcely dare venture an 

 opinion upon a subject so controverted, is, that it is analogous 

 to the heat of friction, that the particles of matter in close 

 approximation and rapid motion inter se evolve heat as a con- 

 tinuation of the motion interrupted by the friction or intesti- 

 nal motion of the particles : heat would thus be produced, 

 whether the resulting compound were of greater or less bulk 

 than the sum of the components, though of course when the 

 compound is of greater bulk less heat would be apparent in 

 neighbouring bodies, the expansion taking place in one of the 

 substances themselves I say in one of them, for it is stated 



