LECT. xv.] HISTORY OF CHEMISTRY 323

accomplished by Wroblewsky,155 whose research may be
regarded as an example of finished skill.

The intimate connection between physics and chemistry,
which is apparent in the matter just dealt with, makes itself
still more clearly manifest when we pass on to thermo-
chemistry. This is a subject which is of equal importance
to both branches, and, moreover, it has been almost entirely
elaborated by men belonging to the two sciences. Lavoisier
and Laplace may be looked upon as the founders of thermo-
chemistry, not only on account of their experimental re-
searches on specific and latent heats and on heats of com-
bustion, but also on account of their masterly definitions,156
and, especially, of the fundamental (if not quite pre-
cisely formulated) principle which they deduce from the
mechanical law of the conversation of energy :—

The heat liberated during combination or change of state
is consumed again during decomposition or return to the
original state, and vice versa™

This principle was enunciated by Hess, in 1840, in
another form which is very important and strictly accurate
for practical thermo-chemistry ;—The evolution of heat
corresponding to any chemical process is the same whether
the process is accomplished in different stages or all at

once.158
Hess established this principle empirically, and he
employed it extensively in order to determine quantities
of heat which were incapable of direct measurement—pro-
ceeding, therefore, in exactly the same manner as is done at
the present day.
The comprehensive researches of Favre and Silber-
mann159 are of great value. These consist, in part, of very
exact thermal determinations, especially of heats of combus-
tion, and, up till about forty years ago, they constituted the
155 Monatshefte. 6, 204. 156 Compare p. 26. 157 Lavoisier, Oeuvres.
2, 287. 158 Pogg. Ann. 50, 385 ; 52> 97- m Ann. Chim. [3] 34» 357 ;
36, i ; 37, 406.