at the
difference between them is of no great importance for ordinary
purposes. But for purposes of definition it would be necessary to take
the mean value of the specific heat _over a given range_ of
temperature, preferably at least 10 deg., rather than the specific
heat _at a point_ which necessitates reference to some formula of
reduction for the rate of variation. The specific heat at 15 deg.
would be determined with reference to the mean over the range 10 deg.
to 20 deg., and that at 20 deg. from the range 15 deg. to 25 deg.
There can be no doubt that the range 10 deg. to 20 deg. is too low for
the accurate thermal regulation of the conditions of the experiment.
The range 15 deg. to 25 deg. would be much more convenient from this
point of view, and a mean temperature of 20 deg. is probably nearest
the average of accurate calorimetric work. For instance 20 deg. is the
mean of the range of the experiments of Griffiths and of Rowland, and
is close to that of Schuster and Gannon. It is readily attainable at
any time in a modern laboratory with adequate heating arrangements,
and is probably on the whole the most suitable temperature to select.

S 17. _Specific Heat of Gases._--In the case of solids and liquids under
ordinary conditions of pressure, the external work of expansion is so
small that it may generally be neglected; but with gases or vapours, or
with liquids near the critical point, the external work becomes so large
that it is essential to specify the conditions under which the specific
heat is measured. The most important cases are, the specific heats (1)
at constant volume; (2) at constant pressure; (3) at saturation pressure
in the case of a liquid or vapour. In consequence of the small thermal
capacity of gases and vapours per unit volume at ordinary pressures, the
difficulties of direct measurement are almost insuperable except in case
(2). Thus the direct experimental evidence is somewhat meagre and
conflicting, but the question of the relation of the specific heats of
gases is one of great interest in connexion with the kinetic theory and
the constitution of the molecule. The well-known experiments of Regnault
and Wiedemann on the specific heat of gases at constant pressure agree
in showing that the _molecular specific heat_, or the thermal capacity
of the molecular weight in grammes, is approximately independent of the
temperature and pressure in case of the more stabl