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 or to one hour’s labour-time. If now, in a c, the working day of 12 hours, we move the point b to b', b c becomes b' c; the surplus-labour increases by one-half, from 2 hours to 8 hours, although the working day remains as before at 12 hours. This extension of the surplus labour-time from b c to b' c, from 2 hours to 8 hours, is, however, evidently impossible, without a simultaneous contraction of the necessary labour-time from a b into a b', from 10 hours to 9 hours. The prolongation of the surplus-labour would correspond to a shortening of the necessary labour; or a portion of the labour-time previously consumed, in reality, for the labourer’s own benefit, would be converted into labour-time for the benefit of the capitalist. There would be an alteration, not in the length of the working day, but in its division into necessary labour-time and surplus labour-time.

On the other hand, it is evident that the duration of the surplus-labour is given, when the length of the working day, and the value of labour-power, are given. The value of labour-power, i.e., the labour-time requisite to produce labour-power, determines the labour-time necessary for the reproduction of that value. If one working hour be embodied in sixpence, and the value of a day’s labour-power be five shillings, the labourer must. work 10 hours a day, in order to replace the value paid by capital for his labour-power, or to produce an equivalent for the value of his daily necessary means of subsistence. Given the value of these means of subsistence, the value of his labour-power is given; and given the value: of his labour-power, the duration of his necessary labour-time is given. The duration of the surplus-labour,