.

Whewell, in his _History of the Inductive Sciences_, says that the
Greeks made no headway in physical science because they lacked
appropriate ideas. The evidence is overwhelming that they were as
observing, as acute, as reasonable as any who live to-day. With this
view, it would appear that the great discoverers must have been men who
started out with appropriate ideas: were looking for what they found.
If, then, one reflects upon the exceeding great difficulty there is in
discovering one new truth, and the immense amount of work needed to
disentangle it, it would appear as if even the most successful have but
indistinct ideas of what is really appropriate, and that their
mechanical conceptions become clarified by doing their work. This is not
always the fact. In the statement of Newton quoted at the head of this
chapter, he speaks of a spirit which lies hid in all gross bodies, etc.,
by means of which all kinds of phenomena are to be explained; but he
deliberately abandons that idea when he comes to the study of light, for
he assumes the existence and activity of light corpuscles, for which he
has no experimental evidence; and the probability is that he did this
because the latter conception was one which he could handle
mathematically, while he saw no way for thus dealing with the other. His
mechanical instincts were more to be trusted than his carefully
calculated results; for, as all know, what he called "spirits," is what
to-day we call the ether, and the corpuscular theory of light has now no
more than a historic interest. The corpuscular theory was a mechanical
conception, but each such corpuscle was ideally endowed with qualities
which were out of all relation with the ordinary matter with which it
was classed.

Until the middle of the present century the reigning physical philosophy
held to the existence of what were called imponderables. The phenomena
of heat were explained as due to an imponderable substance called
"caloric," which ordinary matter could absorb and emit. A hot body was
one which had absorbed an imponderable substance. It was, therefore, no
heavier than before, but it possessed ability to do work proportional to
the amount absorbed. Carnot's ideal engine was described by him in terms
that imply the materiality of heat. Light was another imponderable
substance, the existence of which was maintained by Sir David Brewster
as long as he lived. Electricity and magnetism were imponderable fluids,