274 Dr. van der Kolk on the Mechanical 



this kind takes place in freezing-mixtures, where the cooling is 

 a secondary consequence of the powerful molecular forces ; but 

 similar phenomena do not seem to have been observed in the 

 case of simple combination accompanied by absorption of heat. 



The converse theorem to the above is as follows : — 



If heat is absorbed in the case of a decomposition caused by ele- 

 vation of temperature, an action of the opposite kind will 

 occur on subsequent cooling. 



This theorem cannot be referred to a theoretical basis, • and 

 therefore cannot be looked upon as proved ; it receives, however, 

 frequent confirmation, as the following examples show. 



Carbonate of lime absorbs heat, according to Favre and Sil- 

 bermann, when decomposed by heat, and therefore does not con- 

 tain as much energy as its components taken together. Accord- 

 ingly, lime and carbonic acid combine again on cooling. 



It is worthy of remark that Arragonite, which contains more 

 energy, absorbs scarcely any heat when decomposed. 



It is the same with the slaking of lime. A great deal of heat 

 is given off, and consequently hydrate of lime contains much less 

 energy than its components. By heating, it is decomposed, and 

 it re-forms spontaneously on cooling. 



Heat is evolved in the formation of carbonic acid and water ; 

 consequently these compounds possess less energy than their 

 constituents. Their elements, however, unite only at a high 

 temperature; and if therefore, as stated by Deville, they are 

 decomposed by the mere action of heat, this decomposition must 

 take place at a still higher temperature than that at which they 

 are formed. At low temperatures, nevertheless, no combination 

 occurs, notwithstanding that the components contain much more 

 energy than the products. This is connected with the mode of 

 action of chemical affinity, which at low temperatures is not suffi- 

 ciently powerful to cause the formation of these compounds, 

 although they contain so much less energy. In order that a 

 compound may be produced, two general conditions must be 

 fulfilled : (] ) there must be chemical force or affinity sufficient 

 for combination ; and (2) there must be the energy necessary for 

 combination. There is to a certain extent an analogy between 

 these conditions and those involved in the existence of a galvanic 

 current, which requires not only a difference of tension, but, in 

 addition, the energy needed for the production of the current, 

 and generated by the chemical processes which go on in the 

 battery. One cause alone is in both cases insufficient. Here 

 therefore we may speak of reversible and n on -reversible pro- 

 cesses. If a body is changed by elevation of temperature and 

 evolves heat, it comes into a new condition in which it contains 

 less energy than before, and therefore cannot possibly return of 



