This value in exchange is great, because their beauty, which consists in their luster and their sonorous ring,(722) gives them great value in use; and because, at the same time, their rarity in nature makes the supply of them relatively small,(723) and not susceptible of increase at pleasure.(724) As they contain so large a value in so small a volume, they are adapted to transportation from one place to another, with but little difficulty—a matter of the greatest importance in an instrument of exchange.(725) Hence, it is much easier to keep the demand for them and the supply of them at a level all over the world, than it is the demand and supply of most other commodities. And this all the more as there are not different kinds of gold and silver, but only different qualities of their fineness.(726) It also contributes to the uniformity of their value in exchange, that they minister mainly only to wants of luxury. The most indispensable commodities are subject to the greatest variations in price (see § 103), whereas, in the case of the precious metals, the diversity of uses to which they may be turned contributes greatly to render their value, as instruments of exchange, more equable. If the supply of them be small, gold and silver vessels are less in demand; a part of the old ones are melted down, and _vice versa_. In durability, the precious metals surpass almost all other commodities. They are not at all affected by air or water, and they can be corroded only by very few fluids. Fire may, indeed, change their form, but scarcely in any degree the value of the material of gold, and that of silver very little, and then only when it is subjected to a very powerful blast or draught of air.(727)(728) Hence, while by laying them by, they suffer virtually nothing at all (a most valuable article is an article to deposit savings in), their wear and tear from use may be very much decreased by an admixture with other metals in the proper proportion.(729) This durability contributes largely to keep the price of the precious metals more uniform. By the time that the wheat crop is rightly harvested, the great bulk of the previously stored wheat is, as a rule, consumed; and, therefore, the supply of wheat depends almost entirely on the yield of the last crop. On the other hand, it is probable that there is many a piece of money, the raw material of which was dug from Thracian gold mines in the time of King Philip or from the silver mines of Spain during the reign of Hannibal, in circulation to-day. Compared with the immeasurable stores of gold and silver which have gone on accumulating for thousands of years, the new yield of them, in any one year, is lost like a drop in a bucket. Hence, only when the yield of the mines has continued for a very long time, or when it is exceedingly great or remarkably small, can the price of their products change to any great extent.(730) Even during the revolution in