s should not be placed too
close together, else their heating efficacy will be impaired.

INCRUSTATION AND CORROSION OF BOILERS.

389. _Q._--What is the cause of the formation of scale in marine boilers?

_A._--Scale is formed in all boilers which contain earthy or saline
matters, just in the way in which a scaly deposit, or rock, as it is
sometimes termed, is formed in a tea kettle. In sea water the chief
ingredient is common salt, which exists in solution: the water admitted to
the boiler is taken away in the shape of steam, and the saline matter which
is not vaporizable accumulates in process of time in the boiler, until its
amount is so great that the water is saturated, or unable to hold any more
in solution; the salt is then precipitated and forms a deposit which
hardens by heat. The formation of scale, therefore, is similar to the
process of making salt from sea water by evaporation, the boiler being, in
fact, a large salt pan.

390. _Q._--But is the scale soluble in fresh water like the salt in a salt
pan?

_A._--No, it is not; or if soluble at all, is only so to a very limited
extent. The several ingredients in sea water begin to be precipitated from
solution at different degrees of concentration; and the sulphate and
carbonate of lime, which begin to be precipitated when a certain state of
concentration is reached, enter largely into the composition of scale, and
give it its insoluble character. Pieces of waste or other similar objects
left within a marine boiler appear, when taken out, as if they had been
petrified; and the scale deposited upon the flues of a marine boiler
resembles layers of stone.

391. _Q/_--Is much inconvenience experienced in marine boilers from these
incrustations upon the flues?

_A._--Incrustation in boilers at one time caused much more perplexity than
it does at present, as it was supposed that in some seas it was impossible
to prevent the boilers of a steamer from becoming salted up; but it has now
been satisfactorily ascertained that there is very little difference in the
saltness of different seas, and that however salt the water may be, the
boiler will be preserved from any injurious amount of incrustation by
blowing off, as it is called, very frequently, or by permitting a
considerable portion of the supersalted water to escape at short intervals
into the sea. If blowing off be sufficiently practised, the scale upon the
flues will never be much thicker than a sheet