Energy of Chemical Actions. 281 



to take it as any measure at all where combination is accom- 

 panied by absorption of heat, and the chemical affinity would 

 therefore be a negative quantity. 



In the example of water, already considered, we saw that the 

 affinity, or the tendency to combine, where sufficient energy is 

 present, changes with the temperature, and is accordingly a 

 variable quantity. In like manner it is different in different 

 bodies. If, for example, one acid displaces another from its 

 combinations, the first has a greater affinity for the base than 

 the second. For the most part, however, in such cases as these, 

 as always in precipitation, changes in the distribution of energy 

 take place, whereby the matter is complicated. We can speak 

 of pure effects of affinity only when the quantity of energy is 

 the same before and after the combination. This is always the 

 case, according to Favre and Silbermann, where two salts 

 mutually decompose each other in solution without precipitation. 

 In such experiments, however, it is often very difficult to state 

 how the decomposition occurs. 



It is doubtless by no means easy to give a good definition of 

 affinity. That which is commonly given*, namely, that it is the 

 force which brings about the combination of bodies, and which 

 retains the substances thus formed in their new conditions, has 

 the disadvantage of being ambiguous; we may doubt whether 

 both these meanings always coincide. In what has gone before, 

 the affinity of two bodies has always been taken to mean the 

 total effect which they exert upon each other, account having 

 been already taken of the energy. In this respect the definition 

 is more negative than positive. 



The application of the theory of energy may perhaps give us 

 a clearer insight into this also. The combinations and decom- 

 positions are known experimentally ; they depend upon two 

 causes, one of which, the energy, can be determined ; and it 

 may thus be possible, from the result and the first cause, to 

 acquire a nearer knowledge of the second. But since this affi- 

 nity is different between each pair of substances, such investiga- 

 tions as these demand an exact acquaintance with the specific 

 properties of the various chemical substances. The further 

 development of this subject belongs therefore more to che- 

 mistry than to physics. 



It is worthy of being further remarked that the function of 

 energy in these combinations very often coincides with that of 

 StahPs phlogiston. Just as carbon was considered as containing 

 a large quantity of phlogiston, which escaped when the carbon 

 was burned into carbonic acid, and the amount of which was 

 connected with the heat of combination, so likewise carbon 

 and oxygen in the separate state contain more energy than the 

 * Favre, Theses, &c, p. 1. 



Phil. Mag. S. 4. Vol. 29. No. 196. April 1865. U 



