These figures hold substantially for fresh as well as for sea water, for
the sulphate of lime becomes wholly insoluble in sea water, or in soft
water, at temperatures comprised between 280 deg. and 300 deg. Fahr.

It appears from this that it is simply necessary to heat water up to a
temperature of 250 deg. in order to effect the precipitation of four fifths
of the sulphate of lime it may have contained, or to the temperature of
290 deg. in order to precipitate it entirely. The bearing of these facts on
the purification of feed-waters will appear further on. The explanation
offered to account for the gradually increasing insolubility of sulphate
of lime on heating, is, that the hydrate, in which condition it exists
in solution, is partially decomposed, anhydrous calcic sulphate
being formed, the dehydration becoming more and more complete as the
temperature rises. Sulphate of magnesia, chloride of sodium (common
salt), and all the other more soluble salts contained in natural waters
are likewise precipitated by the process of supersaturation, but owing
to their extreme solubility their precipitation will never be effected
in boilers; all mechanically suspended matter tends naturally to
subside.

Where water containing such mineral and suspended matter is fed to a
steam boiler, there results a combined deposit, of which the carbonate
of lime usually forms the greater part, and which remains more or less
firmly adherent to the inner surfaces of the boiler, undisturbed by the
force of the boiling currents. Gradually accumulating, it becomes harder
and thicker, and, if permitted to accumulate, may at length attain such
thickness as to prevent the proper heating of the water by any fire that
may be maintained in the furnace. Dr. Joseph G. Rogers, who has made
boiler waters and incrustations a subject of careful study, declares