When only hard waters containing much lime are at the service of the manufacturer, turbid solutions are often obtained, which must be filtered before use. In many cases this can be avoided by adding milk of lime to the water in a large vessel; the free carbonic acid unites with the lime, and thus the carbonate of lime, which is only soluble in water containing free carbonic acid, separates as a fine precipitate. Water which has been treated in this way becomes clear after some time, through the deposition of the carbonate of lime; it is then soft water. In order to separate the carbonate of lime in this way, no more than the requisite quantity of milk of lime should be added, so that no lime remains in excess, since this would cause precipitates when salts of lead, copper, iron, etc., were dissolved. In many cases—for example, when lead or barium salts are dissolved—the lime contained in the water can be made harmless by slightly acidifying with acetic or nitric acid. Water which contains sulphate of lime (gypsum) is equally useless for many purposes, as, for example, the solution of lead and barium salts. These metals form insoluble compounds with the sulphuric acid, which render the solution turbid, and can be removed only with difficulty by filtering, on account of their great fineness. They are more easily removed by allowing to settle. Water containing gypsum often contains in addition small quantities of sulphuretted hydrogen. However small the quantities of this gas may be, they still make the water absolutely useless for certain purposes in the manufacture of colours; for example, for the preparation of all pigments containing lead which are obtained by precipitation. The sulphuretted hydrogen forms black compounds with lead, copper, bismuth, mercury and other metals, which impair the brilliance of the colour. A colour made under these conditions is never clean, its hue is injured by the admixture of the black substance. Water which contains much common salt (sodium chloride) is unsuitable for the solution of lead, mercury and silver salts. In consequence of the great affinity of these metals for chlorine, turbid solutions are obtained when their salts are dissolved in water containing common salt. Some waters contain a considerable quantity of iron. Such waters deposit on evaporation, and often on standing exposed to the air, a brown powder of ferric hydrate, which would have considerable influence on the shade of a pigment. White pigments, in the preparation of which such a water is used, have always a brownish tinge; yellow and red pigments are also unfavourably affected.