The solvent power of water is not confined to minerals alone, but, under favorable conditions, will attack and dissolve metal from water pipes or from other metallic surfaces with which it comes in contact. The energy with which water attacks metals depends largely upon the character of the water, the nature of the metal and the amount of free carbonic acid contained in the water. As a rule, soft water attacks and dissolves metals to a greater extent than will hard water, although there are exceptional cases where permanently-hard waters have been known to attack lead pipes with an energy equal to that of soft waters. It is not sufficient that water be soft to cause it to attack metals; there must also be present in the water some oxygen and carbonic acid, either free or in solution. If either the oxygen or the carbonic acid are lacking, the solvent power of the water will be greatly reduced.