Let me start, then, by considering some well established facts in the
physical world which show how the known Law acts under certain known
conditions, and this will lead us on in an intelligible manner to see
how the same Law is likely to work under as yet unknown conditions. If
we had to deal with unknown laws as well as unknown conditions we
should, indeed, be up a gum tree. Fancy a mathematician having to solve
an equation, both sides of which were entirely made up of unknown
quantities--where would he be? Happily this is not the case. The Law is
ONE throughout, and the apparent variety of its working results from the
infinite variety of the conditions under which it may work. Let us lay a
foundation, then, by seeing how it works in what we call the common
course of Nature. A few examples will suffice.

Hardly more than a generation ago it was supposed that the analysis of
matter could not be carried further than its reduction to some seventy
primary chemical elements, which in various combinations produced all
material substances; but there was no explanation how all these
different elements came into existence. Each appeared to be an original
creation, and there was no accounting for them. But now-a-days, as the
rustic physician says in Molière's play of the "Médecin Malgré Lui,"
"nous avons changé tout cela." Modern science has shown conclusively
that every kind of chemical atom is composed of particles of one
original substance which appears to pervade all space, and to which the
name of Ether has been given. Some of these particles carry a positive
charge of electricity and some a negative, and the chemical atom is
formed by the grouping of a certain number of negatively charged
particles round a centre composed of positive electricity around which
they revolve; and it is the number of these particles and the rate of
their motion that determines the nature of the atom, whether, for
instance, it will be an atom of iron or an atom of hydrogen, and thus we