the coldest that art has yet produced--liquid air. The latter is
surrounded by the only yet colder medium, liquid hydrogen, so that no
heat can reach it. Under these circumstances, the radium still gives
out heat, boiling away the liquid air until the latter has entirely
disappeared. Instead of the radiation diminishing with time, it rather
seems to increase.

Called on to explain all this, science can only say that a molecular
change must be going on in the radium, to correspond to 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 to the realm of fact,
and can be decided only by the results of observation and a study of
the laws to which these results may lead.

From the philosophic stand-point, a discussion of this subject which is
of such weight that in the history of thought it must be assigned a
place above all others, is that of Kant in his "Kritik." Here we find
two opposing propositions--the thesis that the universe occupies only a
finite space and is of finite duration; the antithesis that it is
infinite both as regards extent in space and duration in time. Both of
these opposing propositions are shown to admit of demonstration with
equal force, not directly, but by the methods of reductio ad absurdum.
The diff