who are usually
mentioned with Thomson as the great developers of thermo-dynamics, were
both far advanced with their novel studies before they were thirty.
With such a list in mind, we may well agree with the father of inductive
science that "the man who is young in years may be old in hours."

Yet we must not forget that the shield has a reverse side. For was not
the greatest of observing astronomers, Herschel, past thirty-five before
he ever saw a telescope, and past fifty before he discovered the heat
rays of the spectrum? And had not Faraday reached middle life before he
turned his attention especially to electricity? Clearly, then, to make
this phrase complete, Bacon should have added that "the man who is
old in years may be young in imagination." Here, however, even more
appropriate than in the other case--more's the pity--would have been the
application of his qualifying clause: "but that happeneth rarely."

THE FINAL UNIFICATION

There are only a few great generalizations as yet thought out in any
single field of science. Naturally, then, after a great generalization
has found definitive expression, there is a period of lull before
another forward move. In the case of the doctrines of energy, the
lull has lasted half a century. Throughout this period, it is true, a
multitude of workers have been delving in the field, and to the casual
observer it might seem as if their activity had been boundless, while
the practical applications of their ideas--as exemplified, for example,
in the telephone, phonograph, electric light, and so on--have been
little less than revolutionary. Yet the most competent of living
authorities, Lord Kelvin, could assert in 1895 that in fifty years he
had learned nothing new regarding the nature of energy.

This, however, must not be interpreted as meaning that the world has
stood still during these two generations. It means rather that the rank
and file have been moving forward along the road the leaders had
already travelled. Only a few men in the world had the range of thought
regarding the new doctrine of energy that Lord Kelvin had at the middle
of the century. The few leaders then saw clearly enough that if one
form of energy is in reality merely an undulation or vibration among the
particles of "ponderable" matter or of ether, all other manifestations
of energy must be of the same nature. But the rank and file were not
even within sight of this truth for a long time after they h