into work, a large portion is degraded, while only a small
portion is transformed into work. So that while it is very easy
to change all of our mechanical or useful energy into heat, it is
only possible to transform a portion of this heat-energy back
again into work. After each change, too, the heat becomes more
and more dissipated or degraded, and less and less available for
any future transformation. In other words," our authors continue,
"the tendency of heat is towards equalization; heat is par
excellence the communist of our universe, and it will no doubt
ultimately bring the system to an end. .... It is absolutely
certain that life, so far as it is physical, depends essentially
upon transformations of energy; it is also absolutely certain
that age after age the possibility of such transformations is
becoming less and less; and, so far as we yet know, the final
state of the present universe must be an aggregation (into one
mass) of all the matter it contains, i. e. the potential energy
gone, and a practically useless state of kinetic energy, i. e.
uniform temperature throughout that mass." Thus our authors
conclude that the visible universe began in time and will in time
come to an end; and they add that under the physical conditions
of such a universe "immortality is impossible."

Concerning the latter inference we shall by and by have something
to say. Meanwhile this whole speculation as to the final
cessation of cosmical work seems to me--as it does to my friend,
Professor Clifford[3]--by no means trustworthy. The conditions of
the problem so far transcend our grasp that any such speculation
must remain an unverifiable guess. I do not go with Professor
Clifford in doubting whether the laws of mechanics are absolutely
the same throughout eternity; I cannot quite reconcile such a