Of the three classes into which commodities are divided—those absolutely limited in supply, those which may be had in unlimited quantity at a given cost of production, and those which may be had in unlimited quantity, but at an increasing cost of production—the precious metals, being the produce of mines, belong to the third class. Their natural value, therefore, is in the long run proportional to their cost of production in the most unfavorable existing circumstances, that is, at the worst mine which it is necessary to work in order to obtain the required supply. A pound weight of gold will, in the gold-producing countries, ultimately tend to exchange for as much of every other commodity as is produced at a cost equal to its own; meaning by its own cost the cost in labor and expense at the least productive sources of supply which the then existing demand makes it necessary to work. The average value of gold is made to conform to its natural value in the same manner as the values of other things are made to conform to their natural value. Suppose that it were selling above its natural value; that is, above the value which is an equivalent for the labor and expense of mining, and for the risks attending a branch of industry in which nine out of ten experiments have usually been failures. A part of the mass of floating capital which is on the lookout for investment would take the direction of mining enterprise; the supply would thus be increased, and the value would fall. If, on the contrary, it were selling below its natural value, miners would not be obtaining the ordinary profit; they would slacken their works; if the depreciation was great, some of the inferior mines would perhaps stop working altogether: and a falling off in the annual supply, preventing the annual wear and tear from being completely compensated, would by degrees reduce the quantity, and restore the value.