ble." Doubters had indeed, expressed scepticism as to
the validity of Joule's experiments, but the further researches,
experimental and mathematical, of such workers as Thomson (Lord Kelvin),
Rankine, and Tyndall in Great Britain, of Helmholtz and Clausius in
Germany, and of Regnault in France, dealing with various manifestations
of heat, placed the evidence beyond the reach of criticism.

Out of these studies, just at the middle of the century, to which
the experiments of Mayer and Joule had led, grew the new science
of thermo-dynamics. Out of them also grew in the mind of one of the
investigators a new generalization, only second in importance to the
doctrine of conservation itself. Professor William Thomson (Lord Kelvin)
in his studies in thermodynamics was early impressed with the fact that
whereas all the molar motion developed through labor or gravity could
be converted into heat, the process is not fully reversible. Heat can,
indeed, be converted into molar motion or work, but in the process a
certain amount of the heat is radiated into space and lost. The same
thing happens whenever any other form of energy is converted into molar
motion. Indeed, every transmutation of energy, of whatever character,
seems complicated by a tendency to develop heat, part of which is
lost. This observation led Professor Thomson to his doctrine of the
dissipation of energy, which he formulated before the Royal Society of
Edinburgh in 1852, and published also in the Philosophical Magazine the
same year, the title borne being, "On a Universal Tendency in Nature to
the Dissipation of Mechanical Energy."

From the principle here expressed Professor Thomson drew the startling
conclusion that, "since any restoration of this mechanical energy
without more than an equivalent dissipation is impossible," the
universe, as known to us, must be in the condition of a machine
gradually running down; and in particular that the world we live on has
been within a finite time unfit for human habitation, and must again
become so within a finite future. This thought seems such a commonplace
to-day that it is difficult to realize how startling it appeared half a
century ago. A generation trained, as ours has been, in the doctrines
of the conservation and dissipation of energy as the very alphabet
of physical science can but ill appreciate the mental attitude of a
generation which for the most part had not even thought it problematical
whether the sun co