INFLUENCE OF TEMPERATURE ON CHEMICAL REACTION 207 
phenomena point, on the contrary, to the existence of a very Hvely 
atomic motion within the molecule; indeed to motion between the 
parts of the composite atoms (5). There can be no doubt, neverthe- 
less, that the chemist's formulae point in no uncertain way to the actual 
existence of regular periodic motion of some sort, within the molecule. 
The specific heats of substances, therefore, represent the varying 
manner in which an increase in the total energy of a chemical system 
is distributed between this regular motion within the molecules, and 
the irregular movements of the molecules themselves. ^ 
The most fundamental characteristic of a chemical change, other 
than the quantitative relationships which obtain between the relative 
weights of factors and products, is the change in the distribution of 
energy which accompanies the transformation, and which, indeed, 
may be said to cause it. In by far the greater number of cases, this 
redistribution is attended either by the release or by the absorption of 
measurable quantities of heat. Reactions are thus classified as 
exothermic and endo thermic; and there is good reason for believing 
that all are so, though perhaps not in measurable degree, excepting 
only transitions of optical isomers into one another. 
The effect of change in temperature on a chemical reaction is, in 
general, determined: first, by the exothermal or endothermal char- 
acter; second, by the quantities of heat that are released or ab- 
sorbed in the transformation of equivalent weights of substance. 
A study of theinfluence, therefore, must begin with an analysis of 
these relations. 
In the light of the molecular and atomic theories, we must look 
upon this release or absorption of heat in chemical transformations— 
this heat of reaction — as measuring the change in the internal energy 
of the system as it passes from one substance form into another; or, 
more specifically, as equivalent to the difference between the energies 
of atomic motion or stress within the molecules of the reacting sub- 
stances before the change and after. Heats of reaction, therefore, 
2 This statement is, of course, too simple. A measured specific heat may pos- 
sibly include latent heats of transition; but in such cases, the figures do not represent, 
of course, the specific heats of definite substances, and need to be analyzed further 
to yield these values. The energy of molecular rotation and of molecular cohesion 
within the limits defined by any one state of aggregation is in general also included 
in the values of the specific heats; but this may be legitimately considered to be a 
function of the molecular constitution and configuration. - 
