are boiled, the carbonic acid is driven off, and the salts, deprived of
their solvent, are rapidly precipitated in fine crystalline particles,
which adhere tenaciously to whatever surface they fall upon. With
respect to the sulphate of lime, the case is different. It is at best
only sparingly soluble in water, one part (by weight) of the salt
requiring nearly 500 parts of water to dissolve it. As the water
evaporates in the boiler, however, a point is soon reached where
supersaturation occurs, as the water freshly fed into it constantly
brings fresh accessions of the salt; and when this point is reached,
the sulphate of lime is precipitated in the same form and with the same
tenaciously adherent quality as the carbonates. There is, however,
a peculiar property possessed by this salt which facilitates its
precipitation, namely, that its solubility in water diminishes as the
temperature rises. This fact is of special interest, since, if properly
taken advantage of, it is possible to effect its almost complete removal
from the feed-water of boilers,

There is little difference in the solubility of the sulphate of lime
until the temperature has risen somewhat above 212 deg. Fahr., when it
rapidly diminishes, and finally, at nearly 300 deg., all of this salt,
held in solution at lower temperatures, will be precipitated when the
temperature has risen to that point. The following table[1] represents
the solubility of sulphate of lime in sea water at different
temperatures:

Temperature. Percentage Sulph.
Fahr. Lime held in Solution.
217 deg. 0.500
219 deg. 0.477
221 deg. 0.432