f heat is kept up by the
continual contraction of the sun, by mutual gravitation of its parts as
it cools off. This theory has the advantage of enabling us to
calculate, with some approximation to exactness, at what rate the sun
must be contracting in order to keep up the supply of heat which it
radiates. On this theory, it must, ten millions of years ago, have had
twice its present diameter, while less than twenty millions of years
ago it could not have existed except as an immense nebula filling the
whole solar system. We must bear in mind that this theory is the only
one which accounts for the supply of heat, even through human history.
If it be true, then the sun, earth, and solar system must be less than
twenty million years old.

Here the geologists step in and tell us that this conclusion is wholly
inadmissible. The study of the strata of the earth and of many other
geological phenomena, they assure us, makes it certain that the earth
must have existed much in its present condition for hundreds of
millions of years. During all that time there can have been no great
diminution in the supply of heat radiated by the sun.

The astronomer, in considering this argument, has to admit that he
finds a similar difficulty in connection with the stars and nebulas. It
is an impossibility to regard these objects as new; they must be as old
as the universe itself. They radiate heat and light year after year. In
all probability, they must have been doing so for millions of years.
Whence comes the supply? The geologist may well claim that until the
astronomer explains this mystery in his own domain, he cannot declare
the conclusions of geology as to the age of the earth to be wholly
inadmissible.

Now, the scientific experiments of the last two years have brought this
mystery of the celestial spaces right down into our earthly
laboratories. M. and Madame Curie have discovered the singular metal
radium, which seems to send out light, heat, and other rays
incessantly, without, so far as has yet been determined, drawing the
required energy from any outward source. As we have already pointed
out, such an emanation must come from some storehouse of energy. Is the
storehouse, then, in the medium itself, or does the latter draw it from
surrounding objects? If it does, it must abstract heat from these
objects. This question has been settled by Professor Dewar, at the
Royal Institution, London, by placing the radium in a medium next to