a
source of considerable danger by permitting the plates to become
overheated and thereby weakened. When the feed-water is very impure,
therefore, the boilers used are those which permit of very easy
cleaning, such as the Lancashire, Galloway and Cornish types, to the
exclusion of multitubular or water-tube boilers in which thorough
cleaning is more difficult. In other cases, however, the feed-water is
purified by passing it through some type of "softener" before pumping it
into the boiler. Most of the impurities in ordinary feed-water are
either lime or magnesia salts, which although soluble in cold water are
much less so in hot water. In the "softener" measured quantities of
feed-water and of some chemical reagents are thoroughly mixed and at the
same time the temperature is raised either by exhaust steam or by other
means. Most of the impurity is thus precipitated, and some of the
remainder is converted into more soluble salts which remain in solution
in the boiler until blown out. The water is filtered before being pumped
into the boiler. The quantity and kind of chemical employed is
determined according to the nature and amount of the impurity in the
"hard" feed-water.

_Thermal Storage._--In some cases where the work required is very
intermittent, "thermal storage" is employed. Above the boiler a large
cylindrical storage vessel is placed, having sufficient capacity to
contain enough feed-water to supply the boiler throughout the periods
when the maximum output is required. The upper part of this storage
vessel is always in free communication with the steam space of the
boiler, and from the lower part of it the feed-water may be run into the
boiler when required. The feed-water is delivered into the upper part of
the vessel, and arrangements are made by which before it falls to the
bottom of the chamber it runs over very extended surfaces exposed to the
steam, its temperature being thus raised to that of the steam. At times
when less than the normal supply of steam is required for the engine
more than the average quantity of feed-water is pumped into the chamber,
and the excess accumulates with its temperature raised to the
evaporation point. When an extra supply of steam is required, the
feed-pump is stopped and the boiler is fed with the hot water stored in
the chamber. Besides the "storage" effect, it is found that many of the
impurities of the feed become deposited in the chamber, where they are
comparativel