es. The means adopted as a
remedy is usually the installation of zinc plates within the boiler,
which must have positive metallic contact with the boiler metal. In this
way, local electrolytic effects are overcome by a still greater
electrolytic action at the expense of the more positive zinc. The
positive contact necessary is difficult to maintain and it is
questionable just what efficacy such plates have except for a short
period after their installation when the contact is known to be
positive. Aside from protection from such electrolytic action, however,
the zinc plates have a distinct use where there is the liability of air
in the feed, as they offer a substance much more readily oxidized by
such air than the metal of the boiler.

Foaming--Where foaming is caused by organic matter in suspension, it may
be largely overcome by filtration or by the use of a coagulent in
connection with filtration, the latter combination having come recently
into considerable favor. Alum, or potash alum, and iron alum, which in
reality contains no alumina and should rather be called potassia-ferric,
are the coagulents generally used in connection with filtration. Such
matter as is not removed by filtration may, under certain conditions, be
handled by surface blowing. In some instances, settling tanks are used
for the removal of matter in suspension, but where large quantities of
water are required, filtration is ordinarily substituted on account of
the time element and the large area necessary in settling tanks.

Where foaming occurs as the result of overtreatment of the feed water,
the obvious remedy is a change in such treatment.

Priming--Where priming is caused by excessive concentration of salts
within a boiler, it may be overcome largely by frequent blowing down.
The degree of concentration allowable before priming will take place
varies widely with conditions of operation and may be definitely
determined only by experience with each individual set of conditions. It
is the presence of the salts that cause priming that may result in the
absolute unfitness of water for boiler feed purposes. Where these salts
exist in such quantities that the amount of blowing down necessary to
keep the degree of concentration below the priming point results in
excessive losses, the only remedy is the securing of another supply of
feed, and the results will warrant the change almost regardless of the
expense. In some few instances, the impurities