208 FREDERICK BARRY 
which may be measured calorimetrically in a variety of ways and 
recently with a high degree of precision, are data of the most fun- 
damental significance. In interpreting these quantities to be the 
equivalents of intra-molecular energy changes, we have already 
assumed the operation of the law of the conservation of energy. 
Another application of this generalization permits us to assert that 
the internal energy of a molecular system is determined solely by 
its apparent character at any given moment : that no matter by what 
path we proceed in forming one compound from another, the total 
energy change accompanying the transition will be the same. This 
deduction, known as the law of Hess,^ permits us to calculate from 
known data the heat changes which would accompany the formation 
from their elements of a large number of compounds, even in cases 
such that the heat of direct combination cannot, for one reason or 
another, be measured. We have at our command, therefore, a grow- 
ing accumulation of data, which may be made highly accurate by the 
general application of recent experimental methods and which define 
the relative internal energies of a great variety of substances. 
From the results of measurements made at different temperatures, 
these data may now be extended almost at will, to supply a knowledge 
of the alteration of these values with change of temperature. The 
same result may be attained, however, by a further application of the 
first law of energy. This procedure merits attention especially 
because of the light it throws on the nature of the phenomena. Un- 
fortunately the course of reasoning — which is based on the device of 
completing an imaginary reversible cyclic process between two tem- 
peratures — cannot well be followed out here and now. We shall 
content outselves with its result — Kirchoff's law — which, though per- 
fectly general, had in this conection best be stated as applied to a chem- 
3 This principle was enunciated by G. H. Hess in 1840, in somewhat simpler 
terms: to the effect that the heat evolved or absorbed in the formation of any chem- 
ical compound from its elements is always the same, whether the synthesis be accom- 
plished directly or by successive reactions. The original statement was a generaliza- 
tion based directly on experiment. Together with the law of Laplace and Lavoisier 
— formulated fifty years before — which states that the heat evolved in the formation 
of a compound is equal to that absorbed in its decomposition, it stands as one of 
the fundamental bases of chemical energetics. Neither generalization, of course, 
was deduced from the general principle, which in 1840 was not completely formulated. 
It cannot be doubted nevertheless that in the discovery of these regularities the 
principle was foreshadowed. 
