The definition of constant capital given above by no means excludes
 the possibility of a change of value in its elements. Suppose the price
 of cotton to be one day sixpence a pound, and the next day, in consequence
 of a failure of the cotton crop, a shilling a pound. Each pound of the
 cotton bought at sixpence, and worked up after the rise in value, transfers
 to the product a value of one shilling; and the cotton already spun before
 the rise, and perhaps circulating in the market as yarn, likewise transfers
 to the product twice its original value. It is plain, however, that these
 changes of value are independent of the increment or surplus-value added
 to the value of the cotton by the spinning itself. If the old cotton had
 never been spun, it could, after the rise, be resold at a shilling a pound
 instead of at sixpence. Further, the fewer the processes the cotton has
 gone through, the more certain is this result. We therefore find that speculators
 make it a rule when such sudden changes in value occur, to speculate in
 that material on which the least possible quantity of labour has been spent:
 to speculate, therefore, in yarn rather than in cloth, in cotton itself,
 rather than in yarn. The change of value in the case we have been considering,
 originates, not in the process in which the cotton plays the
 part of a means of production, and in which it therefore functions as constant
 capital, but in the process in which the cotton itself is produced. The
 value of a commodity, it is true, is determined by the quantity of labour
 contained in it, but this quantity is itself limited by social conditions.
 If the time socially necessary for the production of any commodity alters
 — and a given weight of cotton represents, after a bad harvest, more labour
 than after a good one — all previously existing commodities of the same
 class are affected, because they are, as it were, only individuals of the
 species, [8] and their value at any given time is measured
 by the labour socially necessary, i.e., by the labour necessary
 for their production under the then existing social conditions.