^73 Dr. Woods on Chemical Combination. 



of particles must be a lessening (for if it were au expansion, the 

 bodies would not come to an insensible distance at all, or at 

 least could not be brought nearer than the very bounds), the 

 heat of chemical action is the necessary simultaneous, equal, and 

 opposite molecular movement or expansion. It follows, also, 

 that every atom, or, not to speak hypothetically, every combining 

 particle of one element must necessarily unite with a particle of 

 the other in the combination of the two bodies ; for as the distance 

 between the particles of the opposite kind is smaller than that 

 between the particles of the same kmd, the relation of the space 

 to the nature of the matter could only be fulfilled by that 

 arrangement. 



If two simple bodies therefore combine, the distance between 

 the elements of each compound particle being less than that 

 between the particles of either body in the uncombincd state, 

 heat or expansion in some other substance must be produced ; 

 just as when the particles of iron, for example, contract or come 

 together from pressure or any other cause, expansion in another 

 substance is the result. I proved in the October Number of 

 the Philosophical Magazine, 1851, that the opposite effect, or 

 cold, is the consequence of the expansion among the particles in 

 the decomposition of a compou.nd. 



The value of external circumstances in chemical combination 

 may be estimated by the fact, that although at first the com- 

 pound particles may absorb the expansion or heat produced by 

 combination, yet other bodies in the end take it up ; and this 

 equal and opposite movement must be modified by them. 



If it be true, then, that the distance between the particles of 

 a body depends on the matter composing it (No. 2), and that 

 chemical combination depends on a lessening of this distance, or 

 rather consists therein, it follows that ajfinitij results from the 

 circumstance that when two bodies are mixed together the nature 

 of the mixed matter requires a smaller distance between the par- 

 ticles than that of either body separately ; and elective affinity 

 is, that when three bodies are brought togetlier at insensible 

 distances, those two which require least distance between the 

 particles always must unite. And as the opposite movement or 

 expansion always accompanies the combination, the heat produced 

 is a measure of the affinity ; or if heat be produced by mixing a 

 simple with a compound body, it shows that the former decom- 

 posed and united with an clement of the latter. Notwithstand- 

 ing that the decomposition of the latter produced cold, yet the 

 combination that takes place at the same time produces more 

 heat than counteracts that cold, bccaiise if that were not the 

 case, the particles of the combining bodies would not have come 

 more closely together than those of the decomposing one, and no 



