24 



Dr. Bevan Lean on the 



acid, the thermal value varies according as one, two, three, 

 etc., molecules of a monacidic base react with one of the acid. 

 Attention should particularly be directed to the fact 

 that the relative energy of combination of the first, second, 

 and third equivalents of a base with an acid are not neces- 

 sarily the same — in fact they are in general different. The 

 following instances may be quoted from Thomsen's work : 



ist NaOH... 

 2nd NaOH... 

 3rd NaOH... 



Heat produced on neutralisation. 



Sulphuric. 



Oxalic. 



Succinic. 



Phosphoric. 



14,75° 

 16,650 



13,85° 

 14,45° 



12,400 



",75° 



14,850 

 12,250 



6,95° 



We have in these numbers the first illustration of that 

 which it is especially desired to bring out in this paper, 

 namely, that the affinity of a polybasic acid is a complex 

 function, and is not measured by a simple multiple of the 

 affinity of one of the acid groups contained within the 

 molecule. 



Although the above numbers sufficiently indicate the 

 basicity of the acids, it is now recognised that the heats of 

 neutralisation of the aqueous solutions of the acids do not 

 represent exactly the quantities of heat liberated by the 

 combinations of the acid with the base. Secondary actions 

 often occur, such as the dissociation of the salts and forma- 

 tion of hydrates, so that the thermal effect observed is only 

 a resultant one. To avoid these sources of error Bertholet 1 

 showed several years ago that it is sufficient to refer all the 

 compounds to the solid state. Some determinations which 

 Massol 2 made in this direction in 1891 are particularly 



1 Ann. Chun. Phys ; 5. IV. 122 and 130. 



2 Comples Rendus ; 1 12, 1062. 



