As mediums of circulation, gold and silver have this advantage over other commodities, that their high specific gravity which condenses much weight in little space, corresponds to their economic specific gravity which condenses relatively much labor-time, i. e. a great quantity of exchange value in a small volume. This insures facility of transport, of transition from hand to hand and from one country to another, the ability to appear as rapidly as to disappear, in short, that material mobility which constitutes the _sine qua non_ of the commodity that is to serve as the _perpetuum mobile_ of the process of circulation. The high specific value of the precious metals, their durability, comparative indestructibility, insusceptibility of oxidation through the action of the air, in the case of gold insolubility in acids except in aqua regia,―all these natural properties make the precious metals the natural material for hoarding. Peter Martyr who seems to have been a great lover of chocolate, remarks, therefore, of the cacao-bags which formed a species of Mexican gold: “O felicem monetam, quae suavem utilemque praebet humano generi potum, et a tartarea peste avaritiae suos immunes servat possessores, quod suffodi aut diu servari nequeat.”[113] The great importance of metals in general in the direct process of production is due to the part they play as instruments of production. Apart from their scarcity, the great softness of gold and silver as compared with iron and even copper (in the hardened state in which it was used by the ancients), makes them unfit for that application and deprives them, therefore, to a great extent, of that property on which the use-value of metals is generally based. Useless as they are in the direct process of production, they are easily dispensed with as means of existence, as articles of consumption. For that reason any desired quantity of them may be absorbed by the social process of circulation without disturbing the processes of direct production and consumption. Their individual use-value does not come in conflict with their economic function. Furthermore, gold and silver are not only negatively superfluous, i. e. dispensable articles, but their aesthetic properties make them the natural material of luxury, ornamentation, splendor, festive occasions, in short, the positive form of abundance and wealth. They appear, in a way, as spontaneous light brought out from the underground world, since silver reflects all rays of light in their original combination, and gold only the color of highest intensity, viz. red light. The sensation of color is, generally speaking, the most popular form of aesthetic sense. The etymological connection between the names of the precious metals, and the relations of colors, in the different Indo-Germanic languages has been established by Jacob Grimm (see his History of the German Language).