54 Prof. S. U. Pickering on 



residual affinity of one of the radicals, in the molecules con- 

 cerned by the excess of water used to dissolve them. When, 

 again, metals are acted on by excess of water (or of weak 

 hydrochloric acid), the heat evolved, though not an absolutely 

 constant quantity, is approximately so in many cases, espe- 

 cially when the cases taken refer to metals belonging to the 

 same group. The chemical reactions involved are of a pre- 

 cisely similar nature in each case ; and the small differences 

 which exist are easily accounted for by the differences in the 

 constitution of the metallic molecules themselves. 



The heat of formation of any given metallic salt in solution 

 is calculated from the heat evolved in reacting on the metal 

 with water, the heat of neutralization, and the heat of forma- 

 tion of the acid and water ; thus 



Na, CI = (Na, aq.) + H, CI, aq. + NaOH aq., HC1 aq. ; 



all of which quantities, except the first (Na,aq.), are identical, 

 or else constants, whatever metal be taken. Hence there, is 

 a constant and comparatively small difference between the 

 heat of formation of solutions of similar salts of different 

 metals : — 



K, CI, aq. -K, N0 3 , aq. = Na, CI, aq. -Na, N0 3 , aq. 



In calculating the heat of formation of salts in the solid 

 state the same equations hold good, except that there must be 

 subtracted the heat of dissolution of the salt in question. As 

 the heat of dissolution of similar salts of different metals is 

 often identical within a thousand or a few hundred calories — a 

 quantity which, though large in proportion to the heat of dis- 

 solution itself, is but insignificant in comparison with the heat 

 of formation of the salt — the same constant differences will 

 very nearly obtain in the heat of formation of the solid salts. 



Again, the substitution of similar radicals in gaseous mem- 

 bers of an homologous series, such as the hydrocarbons and 

 their derivatives, gives rise to the same heat-development. 



All these constants or semi-constants present no difficulties 

 whatever to our minds ; but there is another class of constants 

 where the case is very different. 



Thomsen's second volume of Thermochemical Researches, 

 dealing with the combinations of the non-metals, abounds 

 with instances where the heat evolved in two somewhat 

 similar reactions is a multiple of some common constant : 

 thus, 



Iodic acid. . . . I 2 , 6 , H 2 ,aq. =111,590 = 3 x 37,197, 



Periodic acid . . I, 6 , H 5 ,aq. =184,400 = 5 x 36,880. 

 Hence substitution of I for H 3 = 72,810 = 2 x 36,405. 



