As the use of machinery becomes more general in a particular industry,
 the social value of the product sinks down to its individual value, and
 the law that surplus-value does not arise from the labour-power that has
 been replaced by the machinery, but from the labour-power actually employed
 in working with the machinery, asserts itself. Surplus-value arises from
 variable capital alone, and we saw that the amount of surplus-value depends
 on two factors, viz., the rate of surplus-value and the number of the workmen
 simultaneously employed. Given the length of the working-day, the rate
 of surplus-value is determined by the relative duration of the necessary
 labour and of the surplus-labour in a day. The number of the labourers
 simultaneously employed depends, on its side, on the ratio of the variable
 to the constant capital. Now, however much the use of machinery may increase
 the surplus-labour at the expense of the necessary labour by heightening
 the productiveness of labour, it is clear that it attains this result,
 only by diminishing the number of workmen employed by a given amount of
 capital. It converts what was formerly variable capital, invested in labour-power,
 into machinery which, being constant capital, does not produce surplus-384value.
 It is impossible, for instance, to squeeze as much surplus-value out of
 2 as out of 24 labourers. If each of these 24 men gives only one hour of
 surplus-labour in 12, the 24 men give together 24 hours of surplus-labour,
 while 24 hours is the total labour of the two men. Hence, the application
 of machinery to the production of surplus-value implies a contradiction
 which is immanent in it, since of the two factors of the surplus-value
 created by a given amount of capital, one, the rate of surplus-value, cannot
 be increased, except by diminishing the other, the number of workmen. This
 contradiction comes to light, as soon as by the general employment of machinery
 in a given industry, the value of the machine-produced commodity regulates
 the value of all commodities of the same sort; and it is this contradiction,
 that in its turn, drives the capitalist, without his being conscious of
 the fact, [71] to excessive lengthening of the working-day, in order that he may compensate the decrease in the relative number of
 labourers exploited, by an increase not only of the relative, but of the
 absolute surplus-labour.