the theory of the ether must be true, because it fits in
so well with the enigmatic, contradictory, incomprehensible character of
the universe as revealed to our minds. We can affirm and deny almost
anything of the ether--that it is immaterial, and yet the source of all
material; that it is absolutely motionless, yet the cause of all motion;
that it is the densest body in nature, and yet the most rarified; that
it is everywhere, but defies detection; that it is as undiscoverable as
the Infinite itself; that our physics cannot prove it, though they
cannot get along without it. The ether inside a mass of iron or of lead
is just as dense as the ether outside of it--which means that it is not
dense at all, in our ordinary use of the term.

VI

There are physical changes in matter, there are chemical changes, and
there is a third change, as unlike either of these as they are unlike
each other. I refer to atomic change, as in radio-activity, which gives
us lead from helium--a spontaneous change of the atoms. The energy that
keeps the earth going, says Soddy, is to be sought for in the individual
atoms; not in the great heaven-shaking voice of thunder, but in the
still small voice of the atoms. Radio-activity is the mainspring of the
universe. The only elements so far known that undergo spontaneous change
are uranium and thorium. One pound of uranium contains and slowly gives
out the same amount of energy that a hundred tons of coal evolves in its
combustion, but only one ten-billionth part of this amount is given out
every year.

Man, of course, reaps where he has not sown. How could it be otherwise?
It takes energy to sow or plant energy. We are exhausting the coal, the
natural gas, the petroleum of the rocks, the fertility of the soil. But
we cannot exhaust the energy of the winds or the tides, or of falling
water, because this energy is ever renewed by gravity and the sun. There
can be no exhaustion of our natural mechanical and chemical resources,
as some seem to fear.

I recently visited a noted waterfall in the South where electric power
is being developed on a large scale. A great column of water makes a
vertical fall of six hundred feet through a steel tube, and in the fall
develops two hundred and fifty thousand horse-power. The water comes out
of the tunnel at the bottom, precisely the same water that went in at
the top; no change whatever has occurred in it, yet a vast amount of
power has been taken out of i