he compounds
formed in this way dissolve and wash away; and so you may clean the
foulest boiler or kettle. This is a rather important discovery; for the
effect of fur in a kettle is to oppose the passage of heat, and
therefore to occasion the more fuel to be required to boil water in it,
which is a serious waste and expense when you have a large steam-boiler
to deal with. Dr. Faraday mentions the case of a Government steamer that
went to Trieste, and during the voyage had so much fur formed in her
boiler as to oblige all her coal to be consumed, and then the engineers
were forced to burn spars, rigging, bulkheads, and even chopped cables,
and to use up every shaving of spare timber in the ship. Soot underneath
the kettle, as well as fur inside it, is a hindrance to boiling, as it
is a bad conductor; and that is the reason why one can bear to hold a
kettle of hot water, which is very sooty on its under surface, on the
flat of the hand. So a black kettle doesn't give out its heat readily to
what touches it, and so far it is good to keep water hot; but it gets
rid of heat in another way; as I dare say you know, uncle."

"Eh?" said Mr. Bagges, "why, what?--no--I did know something about it
the other day--but I've such a memory!--and--eh?--no--I've quite
forgotten it."

"By radiation, you know. All warm bodies are constantly giving off rays
of heat, as shining ones are giving off rays of light, although the
heat-rays are invisible."

"How do we know that?" asked Mr. Bagges.

"Get a couple of concave mirrors--a sort of copper basins, polished
inside. Stand them face to face, some yards apart. Put a hot iron
ball--not red hot--in the focus of one mirror. Put a bit of phosphorus
in the focus of the other. The phosphorus will take fire; though without
the mirrors you might place it much nearer the hot iron, and yet it
would not burn. So we know that there are rays of heat, because we can
reflect them as we can rays of light. Some things radiate better than
others. Those that have bright metal surfaces radiate worst, though such
are what are used for reflectors. If their surfaces are blackened or
roughened, they radiate better. A bright kettle gives off fewer rays of
heat than a black one, and so far, is better to keep water hot in. But
then, on the other hand, it yields more heat to the air, or the hob or
hearth that it stands upon--if colder than itself. The bright kettle
gives off heat in one way and the black in another.