the heat it gives out. What that change may be is still a complete
mystery. It is a mystery which we find alike in those minute
specimens of the rarest of substances under our microscopes, in
the sun, and in the vast nebulous masses in the midst of which our
whole solar system would be but a speck. The unravelling of this
mystery must be the great work of science of the twentieth
century. What results shall follow for mankind one cannot say, any
more than he could have said two hundred years ago what modern
science would bring forth. Perhaps, before future developments,
all the boasted achievements of the nineteenth century may take
the modest place which we now assign to the science of the
eighteenth century--that of the infant which is to grow into a
man.





III

THE STRUCTURE OF THE UNIVERSE


The questions of the extent of the universe in space and of its
duration in time, especially of its possible infinity in either
space or time, are of the highest interest both in philosophy and
science. The traditional philosophy had no means of attacking
these questions except considerations suggested by pure reason,
analogy, and that general fitness of things which was supposed to
mark the order of nature. With modern science the questions belong