HARD WATER. By this name is understood such water as will not entirely dissolve soap, but produces a white or greyish coagulum, more or less in quantity in proportion to the hardness of the water. Formerly the hardness of waters was thought to be occasioned by sea-salt: but Dr Home in his Treatise on Bleaching, shows, that this salt can never make water hard, nor indeed can it be done by any perfect neutral, unless mixed in very large quantity with the water. On the other hand, acids of all kinds, imperfect neutrals, and ammoniacal salts, will harden water. The reason of this is, that the alkali of the soap has a less attraction for the oil with which it is united than with the acid, and therefore leaves the oil in order to unite with it; and it is the oil thus separated which we see floating like a curd in hard water when soap is mixed with it. From this theory we have an easy method of softening hard waters; viz. by dissolving in them a quantity of fixed alkali sufficient to absorb the acid, after which it can have no more effect in separating the oil which makes part of the soap. The most common substances by which waters are rendered hard, are selenites and calcareous nitre. From the former they may in good measure be freed by boiling; but not from the latter,
nor from any imperfect salt which readily dissolves in water.