The hardness of water depends to a very large extent upon the nature of the rock through which it percolates, and the extent to which it penetrates. Deep well water obtained from a shaft sunk to a great depth into waterbearing strata is usually more or less hard. Water issuing from springs may he either soft or hard; from granite and the older rocks the water is soft, because it penetrates but little; but in the newer formations, especially magnesian limestone, oolite, lias, chalk, etc., the spring waters are very hard. Water from the surface flowing over pure clay or gravel will be, as a rule, soft, because there is little soluble matter contained therein: but from a shell gravel the water will be hard. Water collected in shallow wells is often very hard, the water percolating readily through the soil and subsoil, and dissolving out the salts contained therein. The salts not precipitated by boiling are removed every time the kettle is emptied; the scale will contain principally the carbonates. In a boiler the case is different, as the concentration of the water by evaporation causes the precipitation of both carbonates and sulphates; but an analysis of the water is better, because there may be present chlorides of calcium and magnesium, which also render the water hard, and may cause trouble in other ways.

These salts are extremely soluble in water, and would not precipitate however long the water was boiled. The deposit inside a kettle would be white if only lime and magnesia were present; but if iron were also present, the deposit would be yellowish or cream-coloured.