rds, the steam must have a
higher temperature than is due to its elastic force, or be in the state of
surcharged steam. The circumstance of the chimney flue passing through the
steam will manifestly surcharge the steam with heat, so that all the
circumstances which are found to accelerate corrosion, are it appears such
as would also induce the formation of surcharged steam.

403. _Q._--Is it the natural effect of surcharged steam to waste away iron?

_A._--It is the natural effect of surcharged steam to oxidate the iron with
which it is in contact, as is illustrated by the familiar process for
making hydrogen gas by sending steam through a red hot tube filled with
pieces of iron; and although the action of the surcharged steam in a boiler
is necessarily very much weaker than where the iron is red hot, it
manifestly must have _some_ oxidizing effect, and the amount of corrosion
produced may be very material where the action is perpetual. Boilers with a
large extent of heating surface, or with descending flues circulating
through the cooler water in the bottom of the boiler before ascending the
chimney, will be less corroded internally than boilers in which a large
quantity of the heat passes away in the smoke; and the corrosion of the
boiler will be diminished if the interior of any flue passing through the
steam be coated with fire brick, so as to present the transmission of the
heat in that situation. The best practice, however, appears to consist in
the transmission of the smoke through a suitable passage on the outside of
the boiler, so as to supersede the necessity of carrying any flue through
the steam at all; or a column of water may be carried round the chimney,
into which as much of the feed water may be introduced as the heat of the
chimney is capable of raising to the boiling point, as under this
limitation the presence of feed water around the chimney in the steam chest
will fail to condense the steam.

404. _Q._--In steam vessels there are usually several boilers?

_A._--Yes, in steam vessels of considerable power and size.

405. _Q._--Are these boilers generally so constructed, that any one of them
may be thrown out of use?

_A._--Marine boilers are now generally supplied with stop valves, whereby
one boiler may be thrown out of use without impairing the efficacy of the
remainder. These stop valves are usually spindle valves of large size, and
they are for the most part set in a pipe which runs across t