Inasmuch as the carbon dioxid which a water naturally holds adds very greatly to the solvent power of the water for mineral substances, it is evident that all those substances which are held in solution in water containing carbon dioxid may be precipitated by simply boiling the water. This form of hardness is caused chiefly by the sulphates of lime and magnesium, bodies which are more or less soluble in water not containing carbon dioxid. The permanent hardness is cured usually by treating the water with carbonate of soda, which converts the calcium sulphate into calcium carbonate and precipitates it. The carbonate of soda also acts upon the Epsom salts, that is, magnesium sulphate, converting it into carbonate and thus precipitating it. The quantity of carbonate of soda employed should be adjusted to the amount of lime and magnesia sulphates which are present.

If waters are excessively alkaline the treatment with bicarbonate of soda may not leave the water in a fit condition either for drinking or for washing purposes. If there is a large quantity of magnesium sulphate in the water the amount of sodium sulphate formed is so large as to interfere with the emulsification of the soap, thus interfering with the washing process. In such conditions the use of freshly slaked lime is to be preferred to that of carbonate of soda. The slaked lime converts the magnesium sulphate into magnesium hydrate, which is precipitated, leaving an equivalent amount of calcium sulphate in solution. If the amount of calcium sulphate is considerable, however, a part of it will be precipitated. Thus, it is generally advisable to treat hard water both with carbonate of soda and slaked lime. When lime and carbonate of soda are both used in softening water, it is advisable that the lime should be used first.

According to some authors the permanent hardness is caused chiefly by magnesium chlorid and calcium sulphate.