§ 5. THE MEASURE OF TEMPERATURE

From the three mechanical units we derive secondary units; as, for
instance, the unit of work or mechanical energy. The kinetic theory
takes temperature, as well as heat itself, to be a quantity of energy,
and thus seems to connect this notion with the magnitudes of
mechanics. But the legitimacy of this theory cannot be admitted, and
the calorific movement should also be a phenomenon so strictly
confined in space that our most delicate means of investigation would
not enable us to perceive it. It is better, then, to continue to
regard the unit of difference of temperature as a distinct unit, to be
added to the fundamental units.

To define the measure of a certain temperature, we take, in practice,
some arbitrary property of a body. The only necessary condition of
this property is, that it should constantly vary in the same direction
when the temperature rises, and that it should possess, at any
temperature, a well-marked value. We measure this value by melting ice
and by the vapour of boiling water under normal pressure, and the
successive hundredths of its variation, beginning with the melting
ice, defines the percentage. Thermodynamics, however, has made it
plain that we can set up a thermometric scale without relying upon any
determined property of a real body. Such a scale has an absolute value
independently of the properties of matter. Now it happens that if we
make use for the estimation of temperatures, of the phenomena of
dilatation under a constant pressure, or of the increase of pressure
in a constant volume of a gaseous body, we obtain a scale very near
the absolute, which almost coincides with it when the gas possesses
certain qualities which make it nearly what is called a perfect gas.