By the same process by which commodities express their values in gold prices, they turn gold into a measure of value i. e. into money. If all of them were to measure their values in silver, wheat, or copper, and therefore express them in the form of silver, wheat or copper prices, then silver, wheat or copper would be measures of value and consequently universal equivalents. In order to appear as prices in circulation, commodities must be exchange values before they enter circulation. Gold becomes the measure of value only because all commodities estimate their exchange value in it. The universality of this relation which is the result of evolution and from which alone springs the function of gold as the measure of value, implies however, that every single commodity is measured in gold, in proportion to the labor-time contained in both; that the actual common measure of the commodity and of gold is labor; or that commodity and gold are passed for each other in direct barter as equal exchange values. How this equalization actually takes place, can not be discussed here when treating of simple circulation. So much, however, is clear, that in countries producing gold and silver, certain quantities of labor-time are directly embodied in definite quantities of gold and silver, while in countries which do not produce gold and silver the same result is reached in a round-about way, by direct or indirect exchange of the commodities of those countries; i. e. a definite portion of average national labor is given for a definite quantity of labor-time, embodied in the gold and silver of the mine-owning countries. In order to be able to serve as a measure of value, gold must be as far as possible a _variable_ value, because it can become the equivalent of other commodities only as an incarnation of labor-time, and the same labor-time is realized in unequal volumes of use-values with the change in the productive power of concrete labor. In estimating all commodities in gold it is only assumed that gold represents a given quantity of labor at a given moment, as was done when the exchange value of any commodity was expressed in terms of the use-value of any other commodity. As for the variations of the value of gold, the law of exchange value formulated above holds good in its case as well. If the exchange value of commodities remains unchanged, then a general rise in their gold prices is possible only in the case of a fall in the exchange value of gold. If the exchange value of gold remains unchanged, a general rise of gold prices is possible only when the exchange value of all commodities rises. The reverse is true in case of a general fall in the prices of commodities. If the value of an ounce of gold falls or rises in consequence of a change in the labor-time required for its production, then the values of all other commodities fall or rise to an equal extent. Thus, the ounce of gold represents after the change, as it did before, a _given_