In order to properly understand the requirements of an effective
feed-water purifier, it will be necessary to understand something of the
character of the impurities of natural waters used for feeding
boilers, and of the manner in which they become troublesome in causing
incrustation or scale, as it is commonly called, in steam boilers. All
natural waters are known to contain more or less mineral matter, partly
held in solution and partly in mechanical suspension. These mineral
impurities are derived by contact of the water with the earth's surface,
and by percolation through its soil and rocks. The substances taken
up in solution by this process consist chiefly of the carbonates
and sulphates of lime and magnesia, and the chloride of sodium. The
materials carried in mechanical suspension are clay, sand, and vegetable
matter. There are many other saline ingredients in various natural
waters, but they exist in such minute quantities, and are generally so
very soluble, that their presence may safely be ignored in treating of
the utility of boiler waters.

Of the above named salts, the carbonates of lime and magnesia are
soluble only when the water contains free carbonic acid.

Our American rivers contain from 2 to 6 grains of saline matter to the
gallon in solution, and a varying quantity--generally exceeding 10
grains to the gallon--in mechanical suspension. The waters of wells and
springs hold a smaller quantity in suspension, but generally carry a
larger percentage of dissolved salts in solution, varying from 10 to 650
grains to the gallon.

When waters containing the carbonates of lime and magnesia in solution