They study the various magnitudes directly
accessible to their observation without busying themselves as to their
essence. They measure quantities of heat and of temperature,
differences of potential, currents, and magnetic fields; and then,
varying the conditions, apply the rules of experimental method, and
discover between these magnitudes mutual relations, while they thus
succeed in enunciating laws which translate and sum up their labours.
These empirical laws, however, themselves bring about by induction the
promulgation of more general laws, which are termed principles. These
principles are originally only the results of experiments, and
experiment allows them besides to be checked, and their more or less
high degree of generality to be verified. When they have been thus
definitely established, they may serve as fresh starting-points, and,
by deduction, lead to very varied discoveries.
The principles which govern physical science are few in number, and
their very general form gives them a philosophical appearance, while
we cannot long resist the temptation of regarding them as metaphysical
dogmas.
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