Affinities of Poly basic Acids. 23 



point of Berzelius a neutral salt was regarded as the com- 

 pound of an acid with a basic oxide, and this combining 

 proportion was considered the rule. 



The conception of the basicity of acids was introduced 

 by Graham's famous research upon the phosphoric acids in 

 1833. He showed that the three phosphoric acids contained 

 one atom of phosphoric acid, POe, in union with different 

 amounts of "basic water," which were replaceable by 

 metallic oxides, and he showed that the different properties 

 of these acids depended upon the different amounts of 

 "basic water" they contained. 



In 1838, after a study of the organic acids — malonic, 

 citric, aconitic, comenic, tartaric, and others — Liebig 

 advanced a Theory of Polybasic acids, and laid down as 

 the criterion of polybasicity the capability of forming salts 

 with different metallic oxides, and he was the first to dis- 

 tinguish between mono-, di-, and tri-basic acids. The theory 

 of polybasic acids was further developed by Laurent and 

 Gerhardt, and later by Wurtz and Kekule. 



As a result of their labours the basicity of an acid has 

 been regarded as determined by the number of stages in 

 which the hydrogen can be replaced ; in other words, by 

 the number of salts it can form with a specified monovalent 

 metal. 



More recently the researches of Thomsen have shown 

 that the basicity of an acid may also be determined by an 

 examination of its Heat of neutralisation, and that the 

 results so obtained agree with the conclusions drawn from 

 the study of the salts of an acid. 



The principle of the thermal method may be stated 

 thus : — the thermal value of the reaction of a monobasic 

 acid with a monacidic base in dilute aqueous solution is 

 independent of the ratio between the number of molecules 

 of acid and base, provided not less than one of base is 

 mixed with one of acid ; whereas in the case of a polybasic 



