The role of prophet is not a scientific one, and the most
firmly established previsions of to-day may be overthrown by the
reality of to-morrow.

Nevertheless, the physicist does not shun an extrapolation of some
little scope when it is not too far from the realms of experiment; the
knowledge of the evolution accomplished of late years authorises a few
suppositions as to the direction in which progress may continue.

The reader who has deigned to follow me in the rapid excursion we have
just made through the domain of the science of Nature, will doubtless
bring back with him from his short journey the general impression that
the ancient limits to which the classic treatises still delight in
restricting the divers chapters of physics, are trampled down in all
directions.

The fine straight roads traced out by the masters of the last century,
and enlarged and levelled by the labour of such numbers of workmen,
are now joined together by a crowd of small paths which furrow the
field of physics. It is not only because they cover regions as yet
little explored where discoveries are more abundant and more easy,
that these cross-cuts are so frequent, but also because a higher hope
guides the seekers who engage in these new routes.

In spite of the repeated failures which have followed the numerous
attempts of past times, the idea has not been abandoned of one day
conquering the supreme principle which must command the whole of
physics.

Some physicists, no doubt, think such a synthesis to be impossible of
realisation, and that Nature is infinitely complex; but,
notwithstanding all the reserves they may make, from the philosophical
point of view, as to the legitimacy of the process, they do not
hesitate to construct general hypotheses which, in default of complete
mental satisfaction, at least furnish them with a highly convenient
means of grouping an immense number of facts till then scattered
abroad.

Their error, if error there be, is beneficial, for it is one of those
that Kant would have classed among the fruitful illusions which
engender the indefinite progress of science and lead to great and
important co-ordinations.

It is, naturally, by the study of the relations existing between
phenomena apparently of very different orders that there can be any
hope of reaching the goal; and it is this which justifies the peculiar
interest accorded to researches effected in the debatable land between
domains hitherto consid