- 
‘ 
J. W. Langley—Chemical Affinity. 371 
This method, indicated by Lavoisier, would seem to be a 
pote one but it has never been followed, so far as I 
now, by anybody unless it may be by Guthrie in his investi- 
‘gation of Cryohydrates.” 
he study of the heat evolved when various elements unite 
with each other and when acids combine with bases has been 
Woods 1851," Favre and Silberman 1853,° J. Thomsen 1853,” 
M. Berthelot 1864 and Alex. Naumann.” 
e work of Thomsen, entitled ‘‘Thermochemische Unter- 
suchungen,” in three volumes, published at Leipsic, in 1882, 
and that of Bertielot, “ Essai de Mechanique Chemique,” in 
wo volumes, Paris, 1879, are models of painstaking and 
exhaustive research. By the labors, chiefly of these two men, 
we now know the thermal values corresponding to many thous- 
ands of chemical reactions. We have learned that the energies 
of a reaction which can be brought about in two methods, 
either in the dry way or by solution, differ in the two cases; 
that of a dibasic or tribasic acid. : 
The most important generalization to be drawn from thermo- 
chemical phenomena is that the work of chemical combination, 
or the total energy involved in any reaction is very largely 
influenced by the surrounding conditions of temperature, pres- 
Sure and volume; and the conclusion they force upon us in 
regard to the nature of affinity is most important, namely, that 
this force in accomplishing work is dependent, like all other 
forces, on the conditions exterior to the reacting system which 
limit the possible amount of change. Affinity is therefore at 
last definitely removed from the category of those mystical 
agents so often invoked by our predecessors in a less critical 
age as belonging to causes which had no correlation with the 
general forces of nature. : 
nder the title Dissociation, St. Claire Deville gave to the 
chemical world in 1857” a new and fruitful method of investi- 
gating the nature of compounds. By determining the tempe- 
tature at which bodies break up or are dissociated he was able 
to perform a purely analytical operation on them not compli- 
cated by the introduction of any extraneous form of matter, 
and since the gradual increase of temperature can be closely 
adjusted and watched, we have in the process of dissociation 
the perfection of an almost ideal analysis. 
Jour. a aed Series, Vou. XXVIII, No. 167.—Nov., 1884. 
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