The nature of incrustation and the evils resulting therefrom having been
stated, it now remains to consider the methods that have been devised
to overcome them. These methods naturally resolve themselves into
two kinds, chemical and mechanical. The chemical method has two
modifications; in one the design is to purify the water in large tanks
or reservoirs, by the addition of certain substances which shall
precipitate all the scale-forming ingredients before the water is fed
into the boiler; in the other the chemical agent is fed into the boiler
from time to time, and the object is to effect the precipitation of the
saline matter in such a manner that it will not form solid masses of
adherent scale. Where chemical methods of purification are resorted to,
the latter plan is generally followed as being the least troublesome. Of
the many substances used for this purpose, however, some are measurably
successful; the majority of them are unsatisfactory or objectionable.

The mechanical methods are also very various. Picking, scraping,
cleaning, etc., are very generally resorted to, but the scale is so
tenacious that this only partially succeeds, and, as it necessitates
stoppage of work, it is wasteful. In addition to this plan, a great
variety of mechanical contrivances for heating and purifying the
feed-water, by separating and intercepting the saline matter on its
passage through the apparatus, have been devised. Many of these are of
great utility and have come into very general use. In the Western States
especially, where the water in most localities is heavily charged
with lime, these mechanical purifiers have become quite indispensable
wherever steam users are alive to the necessity of generating steam with
economy.

Most of these appliances, however, only partly fulfill their intended