50 Dr. Woods on the Heat of Chemical Combination. 



It is that the limit of the approach of the masses is determined 

 by the state of expansion among the particles; or, if two bodies 

 are heated, their particles expand, and, when in approximation, 

 they recede from each other. Many experiments show this fact. 

 The most elegant is that of Dr. Powell's with the two pressed- 

 together pieces of convex glass, in which the alteration of Newton's 

 rings by heat shows the recession of the glasses from each other. 



(26.)" The following, then, is the theory I offer to account for 

 the heat or expansion produced when bodies chemically com- 

 bine. Seeing that the approximation of any particles is always 

 attended by an expansion in others, and vice versa, it might 

 almost be said the two opposite movements being one and the 

 same, — seeing, too, that chemical combinations produce expan- 

 sions in bodies, and chemical decompositions the reverse (12.), 

 am I not justified in saying that the heat of chemical combina- 

 tion, or the expansions it occasions in other bodies, results as a 

 matter of course from the approximating of the particles which 

 form the compound body ? just as the approximation of the par- 

 ticles of one piece of iron causes those of another to separate. 

 The accompanying diagram will explain my meaning. Let 

 and be two particles of oxygen, and I and I two particles of 

 iron. We know that when and O, or a 



I and I, move in the direction of tbe &, ^ ^ «-.. 



arrows at A, that is, when iron or oxygen nn nn ¥ 

 contracts, expansion or heat in other bodies '— ^ ' — ' \ 



is the consequence. Now why not extend i 



the same principle to the case where | * | | * | 

 and I approximate, or the particles move * 



in the direction of the arrows at B ? I 



must remark, that as two pieces of iron at sensible distances 

 coming together do not cause heat or expansion in other bodies, 

 so the oxygen and iron require to form one body before the 

 coming togther of their particles produces a like effect. Bodies 

 may be mechanically mixed, and not give rise to heat or expan- 

 sion ; but the instant they form one body, or are at insensible 

 distances, then they act, as far as the movement of their par- 

 ticles is concerned, exactly as if the one body they form were 

 composed of like particles. 



(27.) The heat of chemical combination is thus looked on as 

 produced exactly in the same way that heat is produced by a 

 simple body whose temperature is higher than surrounding 

 ones; in the latter case the particles come together just as 

 they do when different particles combine in chemical union, 

 but not to the same extent, consequently do not give rise to 

 the same amount of heat or expansion in other bodies (14.). 

 The reason why the particles come together when a chemical 

 compound is formed has nothing to do with our present in- 



