fluid, and who, in 1804, even demonstrated that heat is created by
friction. A few years later he would learn that Charles had enunciated
a capital law on the dilatation of gases; that Pierre Prevost, in
1809, was making a study, full of original ideas, on radiant heat. In
the meantime he would not have failed to read volumes iii. and iv. of
the _Mecanique celeste_ of Laplace, published in 1804 and 1805, and he
might, no doubt, have thought that before long mathematics would
enable physical science to develop with unforeseen safety.

All these results may doubtless be compared in importance with the
present discoveries. When strange metals like potassium and sodium
were isolated by an entirely new method, the astonishment must have
been on a par with that caused in our time by the magnificent
discovery of radium. The polarization of light is a phenomenon as
undoubtedly singular as the existence of the X rays; and the upheaval
produced in natural philosophy by the theories of the disintegration
of matter and the ideas concerning electrons is probably not more
considerable than that produced in the theories of light and heat by
the works of Young and Rumford.

If we now disentangle ourselves from contingencies, it will be
understood that in reality physical science progresses by evolution
rather than by revolution. Its march is continuous. The facts which
our theories enable us to discover, subsist and are linked together
long after these theories have disappeared. Out of the materials of
former edifices overthrown, new dwellings are constantly being
reconstructed.

The labour of our forerunners never wholly perishes. The ideas of
yesterday prepare for those of to-morrow; they contain them, so to