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II. On the Thermal Changes accompanying Basic Substitutions. By Thomas Andrews, 
M.D., M.R.I.A., Professor of Chemistry in the Royal Belfast Institution. Com- 
municated by Michael Faraday, D.C.L., F.R.S., 8$c. 8$c. fyc. 
Received December 14, — Read December 21, 1843. 
In a communication made to the Royal Irish Academy, nearly three years ago, I 
described a series of experiments on the heat evolved during the mutual reaction of 
acids and bases upon one another, from which the general conclusion was deduced, 
that when the influence of all extraneous circumstances is eliminated, the heat is 
determined by the basic and not by the acid element of the combination. Nearly 
at the same time an important memoir was published by M. Hess on thermo-che- 
mistry, in which an opposite result was arrived at, deduced however from a very 
limited number of experiments, and merely announced by its author, as a probable 
generalization, the accuracy of which could only be determined by further researches. 
The principle, as stated by M. Hess, is this, that different bases disengage the same 
quantity of heat in combining with the same acid*. 
In the present state of chemical knowledge we cannot attempt the resolution of 
this problem by direct experiments on the anhydrous acids and bases, even if we 
adopt the hypothesis, no longer universally admitted by chemists, that the proximate 
constituents of neutral salts are the ordinary acids and bases. Experiments per- 
formed with the concentrated acids are not adapted to yield simple results, since the 
mere circumstance of dilution with water produces the evolution of large quantities 
of heat in the case of some acids, and none, or a very slight variation of temperature 
in the case of others. It is for this reason that when an alkaline solution is neutral- 
ized by the addition of an equivalent of nitric acid, the heat disengaged is very dif- 
ferent, according to the state of concentration of the acid ; while the same circum- 
stance produces little or no effect, when the tartaric acid is employed. If we insti- 
tute a further comparison between the results, it will be found that while no simple 
relation exists between the temperatures obtained with different acids in a concen- 
trated state, there is a very close approximation to an equal development of heat 
when the same base is neutralized by any dilute acid. 
In many apparently simple reactions it is difficult to ascertain with certainty alL 
the combinations and decompositions which occur, and our thermal results become 
proportionally difficult to interpret. Even to deduce the heat due to the combina- 
tion of an anhydrous acid and alkali from that evolved when their solutions are 
* Poggendorff’s Annalen, Bd. lii. 107, or Phil. Mag. v. xx. p. 1. 
