CHEMICAL AFFINITY. 145 



(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 b, they would, from their 

 increased proximity, produce an expansive power ultra the 

 volume occupied by the actual chemical compound A and B. 

 The question, however, immediately occurs, why should the 

 volume of the compound be limited and not occupy the full 

 space equivalent to the expanding power induced by the con- 

 traction 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. 



Many compounds evolve heat by decomposition alone, 

 e.g. chloride and iodide of nitrogen, euchlorine, &c. ; this 

 cannot well be from a falling together of the atoms. My 

 own notion of the heat produced by chemical combination 

 though I scarcely dare venture an opinion upon a sub- 

 ject 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 continuation of the 

 motion interrupted by the friction or intestinal motion of the 

 particles : heat would thus be produced, whether the result- 

 ing 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 in books of 

 authority that there is no instance of two or more solids or 



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