both approach nearest to the truth.


I propose to lecture during this term on Heat, and, as our facilities
for experimental work are not yet fully developed, I shall endeavour
to place before you the relative position and scientific connexion of
the different branches of the science, rather than to discuss the
details of experimental methods.

We shall begin with Thermometry, or the registration of temperatures,
and Calorimetry, or the measurement of quantities of heat. We shall
then go on to Thermodynamics, which investigates the relations between
the thermal properties of bodies and their other dynamical properties,
in so far as these relations may be traced without any assumption as
to the particular constitution of these bodies.

The principles of Thermodynamics throw great light on all the
phenomena of nature, and it is probable that many valuable
applications of these principles have yet to be made; but we shall
have to point out the limits of this science, and to shew that many
problems in nature, especially those in which the Dissipation of
Energy comes into play, are not capable of solution by the principles
of Thermodynamics alone, but that in order to understand them, we are
obliged to form some more definite theory of the constitution of
bodies.

Two theories of the constitution of bodies have struggled for victory
with various fortunes since the earliest ages of speculation: one is
the theory of a universal plenum, the other is that of atoms and void.