Dr. Woods on the Heat of Chemical Combination. 51 



quiry, as we are only looking for the cause of the heat pro- 

 duced in chemical combination, not for the cause of the com- 

 bination itself. I have no doubt, however, but that the clue to 

 the proper understanding of chemical combination lies, not m 

 any attractions and repulsions of particles, but in the preserva- 

 tion of the balance between the distances of these particles of 

 which I have spoken above. And if we can extend the laws that 

 regulate the phenomena observed among simple bodies to their 

 combinations, we have, I think, gained much. We divest chemi- 

 cally combining particles of all mysterious influences, such as ca- 

 loric, investing atmospheresof electro-negativeandelectro-positive 

 fluids, &c, and look on them only as particles of matter influenced 

 by external circumstances, and seeing the causeof their adherence, 

 not in attraction, but in the closeness with which they are placed to 

 each other, and the absence of an equal and opposite movement 

 among other particles which should accompany their separation. 



(28.) The expansion among the particles of somebodies when 

 combination takes place, such as in the formation of carbonic 

 acid, the explosion of gunpowder, &c, would seem to show that 

 expansion is sometimes accompanied with heat ; but this expan- 

 sion is effected, not between the bodies combining, as between 

 the oxygen and carbon, but between the compound particles, as 

 between those of the carbonic acid. And this expansion is not 

 without its effect — it produces cold; for the heat produced by 

 combination is not so great when the resulting compound is a 

 gas or liquid, as when it is a solid. For instance, when oxygen 

 combines with hydrogen and forms water, the heat produced 

 amounts to 43 units ; but when it unites with zinc, the oxide of 

 zinc being a solid, the heat amounts to 53 units. When oxygen 

 and phosphorus combine and form a solid compound, the heat 

 evolved is nearly twice as much as when they give rise to a 

 gaseous one. The difference, however, does not, I think, entirely 

 result from the distances between the compound particles of the 

 two oxides of phosphorus being unequal, but, as I have shown 

 (25.), that the amount of expansion between the masses and 

 particles of which they are composed react on each, or deter- 

 mine- each other's limits ; so if the distance between the par- 

 ticles of the compound, water, be greater than that between the 

 particles of oxide of zinc, may it not cause the particles of the 

 oxygen and hydrogen to be further apart than the particles of 

 the oxygen and zinc, and consequently be productive of le a ex- 

 pansion or less heat in other bodies? 



(:l ( .).) My theory of the molecular constitution of matter, and 



lation of the expansions and contractions of bodies, with 



chemical combination, from which 1 make them only differ in 



. and that in the latter case the contractions take place 



between different bodies, leads to Berthollet's view of chemical 



B2 



