The lime and magnesia constituting permanent hardness may be removed by the addition of sodic carbonate (soda ash or crystals); but this is expensive on a large scale, and as an equivalent quantity of sodic sulphate or chloride is left in the water, it is for most purposes of questionable advantage, though in some cases useful for the feed water of boilers. When employed for this purpose, the water should if possible be softened and settled before using, instead of adding the soda in the boiler itself, as is generally done. Soda is the active ingredient of many boiler compositions. For preventing furring, most tanning materials or even waste tan-liquors are very effective, and the danger of any corrosive action is lessened by the addition of a portion of soda ash. So far as is yet known, from the tanning point of view, it is hardly necessary to make any distinction between lime and magnesia, which may be considered simply as "hardness." A hard water probably softens dried hides more slowly, though it is possible that the observed difference may be due in many cases to the lower temperature of wells from which hard water is generally derived. In the actual limes, the hardness of the water can have no appreciable influence, though if sodium sulphide be used alone, a certain waste occurs from temporary hardness, which may render it advisable to add a little lime. It is in the washing of the hides from lime that the influence is first distinctly felt. If limey goods, after unhairing, are placed in a water with much temporary hardness, the same action occurs as in Clark's water softening process, and chalk is deposited in the surface of the hides, making them harsh and apt to "frize" or roughen the grain in scudding, and causing bad colour by combining with the organic acids of the liquors. The common, but not wholly satisfactory, expedient is to add a little lime, or better, a few pailfuls of lime liquor to the water before putting in the hides. The best plan is to use a properly softened water. Permanent hardness is not injurious in this way. The hardness of water, and the dissolved carbonic acid which it contains, are, together with its temperature, the principal factors which determine whether a hide will plump or fall in it. Almost the only accurate investigation of this point has been made by W. Eitner ('Der Gerber,' iii. 183). He placed pieces of hide, unhaired by sweating, and quite flat and fallen, in water for 4 days at a temperature of 46° F. (8° C.), with the following results:-- 1. In distilled water Scarcely at all plumped. 2. " water saturated with CO_{2} Well plumped. 3. " " with lime bicarbonate, 20° German } scale of hardness } Tolerably plump. 4. " " " magnesia bicarbonate, 20° do. do. do. 5. " " " lime sulphate, 20° do. Well plumped. 6. " " " magnesia sulphate, 20° do. Best plumped.