Energy of Chemical Actions. 275 



itself to its previous condition of more energy. This is accord- 

 ingly a non- reversible process. If, on the other hand, heat is 

 taken up during the change, the body in its new condition pos- 

 sesses more energy than before, and hence it may come to pass 

 that it returns spontaneously on cooling to its former state. 



The first case will be of most frequent occurrence, for here 

 the second condition is of necessity always fulfilled. Hence it 

 appears that evolution of heat in combination is the rule, and 

 absorption of heat the exception. 



As examples of reversible processes, we may also mention the 

 phenomena of latent heat in fusion and vaporization. Bodies 

 which have undergone these changes always possess more energy 

 in their new condition than they did previously, and they accord- 

 ingly return of themselves to their former state on cooling. 



This is analogous to the well-known theorem of the dynamical 

 theory of heat, which states that heat can never be transferred 

 from a lower to a higher temperature without the expenditure of 

 work. 



Many long-familiar principles at once take their places among 

 the consequences which follow from these considerations. For 

 instance, (1) the heat of combination remains the same whether 

 the combination takes place suddenly at once or by several 

 stages; (2) the heat of combination of a compound body is 

 in general less than the sum of the heats of its constituents. 



The following examples may be taken as illustrating the ap- 

 plication of the laws of mechaDical energy to the explanation of 

 chemical processes. 



1. It is well known that the electric spark can occasion the 

 combination of gases in two perfectly distinct ways. 



A mixture in equivalent quantities of hydrogen and oxygen, 

 of chlorine and hydrogen, or of carbonic oxide and oxygen, com- 

 bines suddenly with evolution of heat, and in unlimited quantity, 

 under the influence of a single spark. 



Other gaseous mixtures (such, for instance, as nitrogen and 

 oxygen) combine only gradually along the path of the spark 

 itself. No evolution of heat takes place, and the combination 

 of the gases ceases as soon as no more sparks are passed. In 

 the same category we may also place the formation of ozone by 

 means of the electric spark. 



In the first case, the energy of the components exceeds that 

 of the compound. A sufficient quantity of energy is therefore 

 at hand ; but combination does not occur, because the first con- 

 dition (a sufficiently strong affinity) is not fulfilled. Under 

 these circumstances, the electric spark increases the power of 

 affinity : combination takes place between a few atoms of hy- 

 drogen and oxygen, and the heat thereby developed causes 



