and throw off heat with a repellent energy difficult to conceive.

"If two equal balls of thin, bright silver," says Sir John Leslie, "one
of them entirely uncovered and the other sheathed in a case of cambric,
be filled with water slightly warmed and then suspended in a close room,
the former will lose only eleven parts in the same time that the latter
will dissipate twenty parts." The superior heat-retaining capacity which
a clean tin kettle possesses over one that has been allowed to
collect smoke and soot, lies within the compass of the most ordinary
observation.

The experiments of the eminent philosopher just mentioned furnish a
variety of suggestions on the radiation from heated surfaces. He found
that, while the radiating power of clean lead was only 19, it rose to 45
when tarnished by oxidation, that the radiating power of plumbago
was 75, and that of red lead 80. He also discovered that, while the
radiating power of gold, silver, and polished tin was only 12, that
of paper was 98, and lamp black no less than 100. He further says: "A
silver pot will emit scarcely half as much heat as one of porcelain. The
addition of a flannel, though indeed a slow conductor, far from checking
the dissipation of heat, has directly a contrary tendency, for it
presents to the atmosphere a surface of much greater propulsive
energy, which would require a thickness of no less than three folds to
counterbalance."

It is safe to infer from this analogy that the felt covering of boilers
should not only be of considerable thickness, but should be protected
by an external jacketing of some sort; for, though felt is a good
non-conductor, it is a powerful absorber and radiator, more especially
when it has been allowed to contract soot and dust.