the money-capital invested in the purchase of labour-power plays the role of the money-form of the fixed capital, which is but gradually replaced as the active period of the slave’s life expires. Among the Athenians therefore, the gain realised by a slave owner directly through the industrial employment of his slave, or indirectly by hiring him out to other industrial employers (e.g., for mining), was regarded merely as interest (plus depreciation allowance) on the advanced money-capital, just as the industrial capitalist under capitalist production places a portion of the surplus-value plus the depreciation of his fixed capital to the account of interest and replacement of his fixed capital. This is also the rule with capitalists offering fixed capital (houses, machinery, etc.) for rent. Mere household slaves, whether they perform necessary services or are kept as luxuries for show, are not considered here. They correspond to the modern servant class. But the slave system too — so long as it is the dominant form of productive labour in agriculture, manufacture, navigation, etc., as it was in the advanced states of Greece and Rome — preserves an element of natural economy. The slave market maintains its supply of the commodity labour-power by war, piracy, etc., and this rapine is not promoted by a process of circulation, but by the actual appropriation of the labour-power of others by direct physical compulsion. Even in the United States, after the conversion of the buffer territory between the wage-labour states of the North and the slavery states of the South into a slave-breeding region for the South, where the slave thrown on the market thus became himself an element of the annual reproduction, this did not suffice for a long time, so that the African slave trade was continued as long as possible to satisfy the market.
http://www.marxists.org/archive/marx/works/1885-c2/ch20_04.htm
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