Praically all natural water supplies are not chemically pure, generally containing certain amounts of dissolved mineral salts.

The usuallyoccurrinng salts which give water its quality of "hardness "are as follows -

Scale Forming Salts

Carbonate of Lime, Carbonate of Magnesia, Sulphate of Lime. Silica.

The above salts are deposited in all proportions in steam boilers, open boiling vessels, hot water pipes, etc., causing severe damage, waste of fuel, and loss of efficiency.

Non-Scale Forming Salts

Chloride and Nitrate of Lime, Chlroide, Nitrate and Sulphate of Magnesia.

The above salts, either directly or indirectly by their reaction with other substances in the water, give rise to serious trouble, principally from corrosion, which is often rapid and dangerous.

The following is a list of the principal inorganic substances commonly found in natural water supplies :

Calcium Carbonate (CaC03, Marble, Chalk). Carbonate of Lime is only slightly soluble in pure water. It dissolves freely in natural waters containing carbonic acid, forming calcium bicarbonate. On heating, the carbonic acid is driven off, and the normal calcium carbonate is precipitated.

Calcium Sulphate (CaSO4, Gypsum). Sulphate of Lime is soluble to a considerable extent in water, and precipitates at boiler temperatures and pressures as a hard flint-like scale, very resistant to heat and exceedingly difficult to dislodge. It can be removed from water by treatment with Soda Ash (Sodium Carbonate) which precipitates Calcium Carbonate, leaving harmless Sodium Sulphate in the water.

Calcium Chloride (CaCl2). Is very soluble and will not cause incrustation. Its chief objection in a water is that it may decompose sulphates, forming Calcium Sulphate.

Calcium Nitrate (Ca(N03)a). Does not form scale but reacts with sulphates, forming calcium sulphate. Calcium Nitrate is also corrosive.

Magnesium Carbonate (MgC03).

Behaves similarly to calcium carbonate, the normal carbonate being sparingly soluble, and the bicarbonate much more so.

Magnesium Sulphate (MgSO4

Epsom Salts). Is very soluble in water and does not of itself form scale, but if the water also contains calcium carbonate, these two ingredients, when heated, may react upon each other and form magnesium carbonate and calcium sulphate. Magnesium sulphate with sodium chloride (which is present in all natural waters) is a potential source of magnesium chloride (a very corrosive salt).

Magnesium Chloride (MgCl2). Does not form scale, but is most objectionable in boiler feed, as at boiler temperatures and pressures it decomposes, forming hydrochloric acid, which attacks and corrodes the boiler.

Sodium Sulphate (Na2SO4. Glauber's Salt). A neutral salt which neither forms scale nor corrodes.

If in considerable quantity it has a tendency to cause foaming.

Sodium Chloride (NaCl, Ordinary Salt). Behaves similarly to sodium sulphate.

Iron (Fe). Usually exists in water as bicarbonate ; on standing, the carbonic acid escapes and the iron precipitates as oxide. Iron in acid water may exist in the form of sulphate. In either form iron yields readily to treatment.

Alumina (Al2O3). Is found to a slight extent in most waters.

Silica (SiO2). Exists in all water to a limited extent, and is of itself of comparatively little importance unless present in appreciable quantity, when it unites with other ingredients to form an extremely hard scale.

Carbon Dioxide (CO2, Carbonic Acid). Is found in practically all natural waters and holds carbonates of iron, calcium and magnesium in solution as bicarbonates.

Water softening by the precipitation method is, above all things, a precise chemical operation, which involves adding to the water calculated quantities of certain reagents (lime, soda, barium compounds, alumina, etc.), capable of interacting with the impurities present in the water to produce insoluble precipitates. These are then removed by settlement and filtration, so that the water after treatment contains only a residuum of its former hardness.

It is obvious that in such a procedure, the first essential is accurate automatic measurement of both chemicals and water, and the second is the capability of ready adjustment of the measuring gear employed, so as to meet the variations which frequently occur in the initial hardness of water supplies, or in the strengths of the commercial reagents used. There is little doubt that the primary cause of the comparative failure of many existing types of water softeners is the fact that in these plants such exact measurement is not attempted, the proportioning of chemicals and water being roughly affected by dividing slides, lift-pipes, floats, cups and scoops, etc., none of which can be pretended to accomplish close measurement. The Lassen Hjort System of water softening provides for the rigidly exact automatic measurement of every gallon of water passed through the plant, and its immediate admixture with a similarly measured quantity of reagent of a specified strength. The treatment of each individual water is a separate problem in itself, and any attempt to apply one stereotyped method to all waters is foredoomed to failure. Every water has its own reaction time ; each water reacts better to a particular proportion of reagents than to any other ; some waters clarify rapidly ; some will not clarify at all without special treatment ; and the successful designing of plant to deal thoroughly with these varying aspects of water purification calls for the highest degree of expert chemical and engineering knowledge and wide practical experience.