ential--Motions, kinds and transformations
of--Mechanical, molecular, and atomic--Invention of
Ethers, Faraday's conceptions.

'And now we might add something concerning a most subtle spirit
which pervades and lies hid in all gross bodies, by the force
and action of which spirit the particles of bodies attract
each other at near distances, and cohere if contiguous, and
electric bodies operate at greater distances, as well repelling
as attracting neighbouring corpuscles, and light is emitted,
reflected, inflected, and heats bodies, and all sensation is
excited, and members of animal bodies move at the command of
the will.'--NEWTON, _Principia_.

In Newton's day the whole field of nature was practically lying fallow.
No fundamental principles were known until the law of gravitation was
discovered. This law was behind all the work of Copernicus, Kepler, and
Galileo, and what they had done needed interpretation. It was quite
natural that the most obvious and mechanical phenomena should first be
reduced, and so the _Principia_ was concerned with mechanical principles
applied to astronomical problems. To us, who have grown up familiar with
the principles and conceptions underlying them, all varieties of
mechanical phenomena seem so obvious, that it is difficult for us to
understand how any one could be obtuse to them; but the records of
Newton's time, and immediately after this, show that they were not so
easy of apprehension. It may be remembered that they were not adopted in
France till long after Newton's day. In spite of what is thought to be
reasonable, it really requires something more than complete
demonstration to convince most of us of the truth of an idea, should the
truth happen to be of a kind not familiar, or should it chance to be
opposed to our more or less well-defined notions of what it is or ought
to be. If those who labour for and attain what they think to be the
truth about any matter, were a little better informed concerning mental
processes and the conditions under which ideas grow and displace others,
they would be more patient with mankind; teachers of every rank might
then discover that what is often called stupidity may be nothing else
than mental inertia, which can no more be made active by simply willing
than can the movement of a cannon ball by a like effort. We _grow_ into
our beliefs and opinions upon all matters, and scientific ideas are no
exceptions