_Permanent hardness_ of water is generally caused by sulphates of lime and magnesia, and more rarely by chlorides and nitrates. As none of these can be precipitated by lime, permanent hardness cannot be removed by Clark’s process, nor can it produce the injurious effect on limed hides which have been attributed to temporary hardness. Neither can the lime and magnesia present combine with the tannins if used for leaching, since they are already fixed by stronger acids, and at most can only act injuriously by slightly lessening the solubility of the tannins. Even this effect cannot be regarded as proved, though it deserves further investigation.[63] Permanent hardness is therefore of little moment as regards the ordinary uses of the tannery, though it has considerable influence in some of the processes of dyeing, and acts very injuriously where soap is used for scouring, as in the washing of sheep-skins for wool mats, since each part of lime reckoned as carbonate destroys at least twelve parts of pure soap (sodium stearate or oleate), producing a sticky and insoluble lime-soap which adheres to the fibre. In sole-leather tanning, permanent hardness is sometimes advantageous, especially if it be due to calcium and magnesium sulphates, and Vignon recommended that sulphuric acid should be added to the water before use in quantity sufficient to exactly neutralise the carbonates which cause temporary hardness, as magnesium and calcium sulphates are not injurious, but tend to plump the hides. It must be remembered, however, that the carbonic acid liberated may still have prejudicial effects on limed hides. [63] Recent investigations by Nihoul (‘Influence de la nature de l’eau sur l’extraction des matières tannantes,’ Bulletin de la Bourse aux Cuirs de Liège, Sept. 1901) on the tanning waters of Belgium seem to show that permanent hardness is more injurious in the extraction of tannin than has generally been supposed.