Energy of Chemical Actions. 271 



water-vapour contains less than was contained in its constituents 

 will be exactly equal to the amount of heat set free. It will be 

 understood that the combination of oxygen and hydrogen is here 

 supposed to take place in a closed space, and therefore without 

 any development of external work; otherwise this must also be 

 taken into account. 



If aqueous vapour is decomposed at the same temperature, as 

 much energy must be given to it as was set free by the combi- 

 nation. 



Accordingly, two cases are possible : either the body possesses 

 more energy than its constituents, or it contains less. 



In the first case, heat is produced by the decomposition of the 

 body; in the second case it is destroyed. 



Heat is therefore set free when we heat a compound of the 

 first class until decomposition begins. The components will 

 then no longer recombine spontaneously by subsequent cooling, 

 for the quantity of energy contained in them is not sufficient for 

 the formation of the compound body at the same temperature. 

 Combination would be possible only if one of two cases occurred : 

 either the body must take heat from surrounding matter, whose 

 temperature never exceeds, but is at most equal to its own ; or 

 there must be a sudden cooling of the body formed. So far as 

 I know, such a case of cooling has not hitherto presented itself. 



The law developed above may accordingly be enunciated thus : 



Bodies which evolve heat when decomposed by elevation of tem- 

 perature are not reproduced by subsequent cooling. 



Hence follows, as a direct consequence, the connexion of the 

 above-named properties. Let us test this law by means of Favre 

 and Silbermamr's results*. 



1. Nitrous oxide evolves heat when decomposed. There exists 

 therefore less energy in nitrogen and oxygen when they are sepa- 

 rate, than when they are combined in the form of this compound ; 

 accordingly they do not recombine when afterwards cooled. 



This conclusion is not affected by Favre and Silbermann's 

 explanation that ozone comes into play here. 



2. Binoxide of hydrogen evolves heat on decomposition ; and 

 hence again there exists more energy in H 2 O 2 , than in H 2 

 and O when they are separate. By cooling, the combination is 

 not reproduced. 



3. Oxide of silver appears likewise to give off heat when de- 

 composed ; and it is not formed again by cooling. 



4. According to Favre f, hypochlorous and chloric acids evolve 



* Annates de Chimie et de Physique, 3 ser. vol.xxxvi. p. 1. 

 t Theses presentees a la Faculte des Sciences de Paris (Mallet-Bache- 

 lier, 1853). In this work (p. 52), Favre has already remarked that 



