282 On the Mechanical Energy of Chemical Actions. 



carbonic acid formed from them; and this difference is, as it 

 were, indicated by the heat of combination. 



It is obviously impossible to do more in this short communi- 

 cation than to point out the general features of the application 

 of the theory of energy to a few cases of chemical action ; but 

 what has been said may suffice to prove the importance of this 

 theorem in the region of chemical phenomena. 



Maestricht, March 1864. 



Postscript to the foregoing Memoir*. 



The principle that the heat of combination cannot be taken as 

 a measure of affinity can perhaps be rendered intelligible as fol- 

 lows. When a freely falling body comes suddenly to rest, we 

 must assume that its vis viva, \mv 1 i is converted into heat, pro- 

 vided no other effects are produced. Supposing we could mea- 

 sure this quantity of heat accurately, as well as the final velocity 

 of the falling body, each experiment would furnish a demonstra- 

 tion of the principle of the conservation of energy; and this 

 result would be the same in whatever part of the earth the expe- 

 riment was made. The variations and other properties of gravity 

 upon the earth would nevertheless remain quite unknown to us. 



This case is analogous to the one we are considering. We 

 measure the heat of combination, which may be taken as the 

 measure of the energy or vis viva expended, that is to say, of a 

 product one factor of which is the affinity or force of chemical 

 attraction, and the other the change of position which occurs 

 under the influence of this force. This change of position is no 

 doubt much more complicated than in the case above considered, 

 inasmuch as the force cannot be regarded as by any means inde- 

 pendent of the mutual positions of the molecules. But as the 

 positions of the molecules are entirely unknown, nothing can be 

 predicated of the forces ; and as the properties of gravity in the 

 case supposed above, so here the chemical forces remain quite 

 unknown to us. 



The action of these forces manifests itself in the combinations 

 which occur as a consequence of chemical affinity. A stronger 

 acid displaces a weaker one from combination because the affinity 

 of the former for the base is greater than that of the latter. Now 

 in Favre and Silbermann's experiments we certainly find in 

 general the greatest heat of combination developed by the strong- 

 est combinations; in some cases, however, we find the opposite. 

 There is nothing at all strange in this ; for there is, strictly speak- 

 ing, no connexion at all between the two magnitudes, and we 

 have reason to be surprised rather at the prevailing agreement 

 than at the occurrence of exceptions. 

 July 1864. 



* Poggendorff's Annalen, vol. cxxii. p. 658 (August 1864). 



