The word “sacrifice” conveys a just idea of what the laborer undergoes, and it corresponds to the abstinence of the capitalist. Of the other requisite—appropriate natural objects—it is to be remarked that some objects exist or grow up spontaneously, of a kind suited to the supply of human wants. There are caves and hollow trees capable of affording shelter; fruits, roots, wild honey, and other natural products, on which human life can be supported; but even here a considerable quantity of labor is generally required, not for the purpose of creating, but of finding and appropriating them. Of natural powers, some are unlimited, others limited in quantity. By an unlimited quantity is of course not meant literally, but practically unlimited: a quantity beyond the use which can in any, or at least in present circumstances, be made of it. Land is, in some newly settled countries, practically unlimited in quantity: there is more than can be used by the existing population of the country, or by any accession likely to be made to it for generations to come. But, even there, land favorably situated with regard to markets, or means of carriage, is generally limited in quantity: there is not so much of it as persons would gladly occupy and cultivate, or otherwise turn to use. In all old countries, land capable of cultivation, land at least of any tolerable fertility, must be ranked among agents limited in quantity. Coal, metallic ores, and other useful substances found in the earth, are still more limited than land. For the present I shall only remark that, so long as the quantity of a natural agent is practically unlimited, it can not, unless susceptible of artificial monopoly, bear any value in the market, since no one will give anything for what can be obtained gratis. But as soon as a limitation becomes practically operative—as soon as there is not so much of the thing to be had as would be appropriated and used if it could be obtained for asking—the ownership or use of the natural agent acquires an exchangeable value. Rich lands in our Western Territories a few years ago could be had practically for the asking; but now, since railways and an increase of population have brought them nearer to the markets, they have acquired a distinct exchange value. The value of a commodity (it may be anticipated) is the quantity of other things for which it can be exchanged. When more water-power is wanted in a particular district than there are falls of water to supply it, persons will give an equivalent for the use of a fall of water. When there is more land wanted for cultivation than a place possesses, or than it possesses of a certain quality and certain advantages of situation, land of that quality and situation may be sold for a price, or let for an annual rent. § 2. The Second Requisite of Production, Labor.