Evolution of the Doctrine of Affinity. 507 



evolved by the sum of the impacts should be a measure of the 

 affinity. But, from the first, difficulties have to be surmounted 

 in accepting this view. One of the greatest is that it seldom 

 occurs that a compound is formed through the union of isolated 

 atoms ; in almost all cases the atoms themselves have to be 

 liberated from compounds in which they have existed in a 

 state of combination. As this liberation must be accompanied 

 by gain of energy (i. e. by absorption of heat), while the 

 formation of the new compound gives rise to loss of energy 

 (i. e. a heat-evolution), it happens that, as a rule, only the 

 difference between the absorption and emission is manifested 

 externally. Hence this doctrine of affinity can deduce from 

 observations, not the absolute value of either affinity, but only 

 the amount by which the one is greater than the other. 

 Many other difficulties present themselves ; especially the 

 fact that, along with chemical changes^ physical changes 

 (alteration of the state of aggregation, of the volume, and so 

 on) occur simultaneously, and are themselves accompanied by 

 an emission or by an absorption of heat. Moreover, thermo- 

 chemical experiments are by no means easy of execution, and 

 are subject to many sources of error ; hence it is not to be 

 wondered that slow progress was made in developing the 

 thermochemical doctrine of affinity. The more numerous 

 the observations, and the greater their accuracy, the greater 

 the number of instances in which theory and experiment 

 failed to display coincidence. It has been frequently observed 

 that a much larger evolution of heat occurs on neutralizing an 

 evidently weak acid than a strong one capable of expelling 

 the weak one more or less completely from its compounds 

 with bases. The expulsion, in such cases, is attended by an 

 absorption, not an evolution, of heat. Similar facts have also 

 been noticed in not a few other chemical reactions, which must 

 be, and have been, regarded as produced by the action of 

 affinity; e.g., the formation of the ethereal salts of organic 

 acids by their action on the alcohols*. Many attempts have 

 been made to explain reactions which are accompanied by 

 negative heat-changes, and to bring them into unison with 

 the thermal doctrine of affinity. But as all such attempts 

 have been unsuccessful, the fundamental hypothesis of the 

 doctrine loses much of its probability. And consequently its 

 most ardent supporter, M. BertheJot, has relinquished his 

 assertion that, by the action of the affinities of all the substances 

 partaking in a reaction, those compounds are always formed 

 which are accompanied by the greatest evolution of heat. He 

 has modified his statement to this : that there is present " a 

 * J. Thomsen, Thermoch. Untersuch. iv. p. 383. 

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