The rapidity of circulation being given, the volume of currency is simply determined by the prices of commodities. Hence, prices are not high or low, because there is more or less money in circulation, but on the contrary, there is more or less money in circulation, because prices are high or low. This is one of the most important laws, whose demonstration in detail by means of the history of prices constitutes perhaps the only merit of the post-Ricardian English Political Economy. If experience shows, that the level of metallic circulation or the mass of gold and silver in circulation in a given country is subject to temporary ebbs and tides and very violent ones at times,[75] but on the whole remains stationary for long periods, the deviations forming but small oscillations about the average level, this is explained by the antagonistic nature of the circumstances which determine the quantity of money in circulation. Their simultaneous modifications neutralize their effects and leave everything where it was before. The law, that with a given rapidity of circulation of money and a given total sum of prices of commodities the quantity of the circulating medium is determined, may also be expressed as follows. If the exchange values of commodities and the average rapidity of their metamorphoses are given, the quantity of gold in circulation depends on its own value. If, therefore, the value of gold, i. e. the labor-time necessary for its production, should rise or fall, the prices of commodities will rise or fall in inverse ratio, and corresponding to that rise or fall of prices, the rapidity of circulation remaining the same, a larger or smaller quantity of gold would be required to keep the same volume of commodities in circulation. The same change would occur, if the old standard of value were superseded by a more or less valuable metal. Thus, Holland required from fourteen to fifteen times as much silver as it had previously required gold, in order to circulate the same volume of commodities, when out of tender regard for the government creditors and out of fear of the effects of the discoveries in California and Australia it substituted silver for gold money. From the fact that the quantity of gold in circulation depends on the variable sum total of prices of commodities and the varying rapidity of circulation, it follows that the volume of the circulating medium must be capable of contraction and expansion; in short, that according to the requirements of circulation, gold must now enter, now leave the sphere of circulation in its capacity of a medium of circulation. How the circulation process itself realizes these conditions, we shall see later on. C. COIN AND SYMBOLS OF VALUE.