I can by no means, therefore, agree with the author, when he says, speaking of the precious metals, that, “in regard to measuring or comparing value, there is no operation that can be intelligibly described, or consistently imagined, but may be performed by the media of which we are in possession.”[73] Surely, to measure the relative power of a commodity over labour and the mass of other commodities, at different and distant times, is an operation which may be both consistently imagined, and intelligibly described; yet it is quite certain, that, in regard to distant periods, the precious metals will not perform this well. Would the author himself venture to say, that the general power of purchasing possessed by an ounce of silver in the time of Edward the Third, was not very much greater than the general power of purchasing possessed by an ounce of silver in the time of George the Fourth; or, that the same quantity of agricultural labour, at these two periods, would not much more nearly have represented the same general power of purchasing? The author seems equally unfortunate when he launches out in praise of the precious metals as a measure of value, as when he says that they do not perform this function better than corn. It will be observed that, in speaking of the values of commodities, at different periods, as meaning their different powers of purchasing at those periods, the kind of value referred to is, exclusively, value in exchange. And, in reference to value in exchange, exclusively, it appears to be of the utmost importance to the language of political economy, to distinguish between the power of purchasing generally, and the power of purchasing any one commodity. But it must not be imagined that when the estimation in which a commodity is held at different periods is referred to, as determined at the time by the state of the supply compared with the demand, and ordinarily by the natural and necessary conditions of its supply, or by the elementary costs of its production, which are equivalent expressions, that value in exchange is lost sight of. Yet the author is continually falling into this kind of misapprehension, and into a total forgetfulness of his first account of the meaning of value, in his examination of Mr. Ricardo’s views, as to the uses of a measure of value, in which, he says, a singular confusion of thought is to be discovered.[74]