CHEMICAL AFFINITY. 421 



" The solubility or insolubility of principles, at the tem- 

 perature of any experiment, has likewise tended to mislead 

 chemists of inferior accuracy, who have deduced conse- 

 quences from the first effects of their experiments. It is 

 evident that many separations may ensue without precipita- 

 tion, because this circumstance does not take place unless 

 the separated principle be insoluble or nearly so. The soda 

 cannot be precipitated from a solution of sulphat of soda 

 by the addition of potash, because of its great solubility ; 

 but, on the contrary, the new compound itself, or sulphat 

 of potash, which is much less soluble, may fall down, if 

 there be not enough water present to suspend it. No 

 certain knowledge can therefore be derived from the ap- 

 pearance or the want of precipitation, unless the products 

 be carefully examined." 



The preceding paragraphs might almost have been 

 copied from the latest modern text-book, and the errors 

 against which the author warns his contemporaries still 

 continue to mislead "chemists of inferior accuracy". The 

 principle involved in the distribution of a base between two 

 acids, i.e., the general principle of balanced action, affords, 

 as we shall see, one of the best means of determining the 

 relative affinity of the competing substances. 



Some thirty or forty years ago great hopes were enter- 

 tained that thermochemistry, as then understood, would 

 furnish us with a true measure of chemical affinity. The 

 energy lost or gained in a chemical reaction can easily be 

 measured in terms of heat, and the determination of these 

 heats of reaction formed almost the sole problem of thermo- 

 chemistry until within quite recent years. It was thought that 

 the chemical force or affinity causing the action must in 

 some way be proportional to the thermal effect of the action. 

 But this is merely a recrudescence of the old dog-Latin 

 fallacy causa cequat effectum, and has no theoretical founda- 

 tion. Berthelot gave expression to this idea in his principle 

 of maximum work, which states that in a chemical change 

 those substances are produced whose formation occasions 

 the greatest development of heat. Although there is no 

 doubt that this principle roughly represents the facts when 



