er were used in
them. The thin film of scale spread over the parts of a marine boiler
situated beneath the water, effectually protect them from corrosion; and
when the other parts are completely worn out the flues generally remain so
perfect, that the hammer marks upon them are as conspicuous as at their
first formation. The operation of the steam in corroding the interior of
the boiler is most capricious--the parts which are most rapidly worn away
in one boiler being untouched in another; and in some cases one side of a
steam chest will be very much wasted away while the opposite side remains
uninjured. Sometimes the iron exfoliates in the shape of a black oxide
which comes away in flakes like the leaves of a book, while in other cases
the iron appears as if eaten away by a strong acid which had a solvent
action upon it. The application of felt to the outside of a boiler, has in
several cases been found to accelerate sensibly its internal corrosion;
boilers in which there is a large accumulation of scale appear to be more
corroded than where there is no such deposit; and where the funnel passes
through the steam chest the iron of the steam chest is invariably much more
corroded than where the funnel does not pass through it.

402. _Q._--Can you suggest no reason for the rapid internal corrosion of
marine boilers?

_A._--The facts which I have enumerated appear to indicate that the
internal corrosion of marine boilers is attributable chiefly to the
existence of surcharged steam within them, which is steam to which an
additional quantity of heat has been communicated subsequently to its
generation, so that its temperature is greater than is due to its elastic
force; and on this hypothesis the observed facts relative to corrosion
become to some extent explicable. Felt, applied to the outside of a boiler,
may accelerate its internal corrosion by keeping the steam in a surcharged
state, when by the dispersion of a part of the heat it would cease to be in
that state; boilers in which there is a large accumulation of scale must
have worked with the water very salt, which necessarily produces surcharged
steam; for the temperature of steam cannot be less than that of the water
from which it is generated, and inasmuch as the boiling point of water,
under any given pressure, rises with the saltness of the water, the
temperature of the steam must rise with the saltness of the water, the
pressure remaining the same; or, in other wo