The variable circulating capital expended in production can serve afresh in the process of circulation only to the extent that the product in which its value is reproduced has been sold, converted from a commodity-capital into a money-capital, in order to be once more laid out in payment of labour-power. But the same is true of the constant circulating capital (materials of production) invested in production, the value of which reappears in the product as a portion of its value. What these two portions — the variable and the constant part of the circulating capital — have in common and what distinguishes them from the fixed capital is not that the value transferred from them to the product is circulated by the commodity-capital, i.e., through the circulation of the product as a commodity. One portion of the value of the product, and thus of the product circulating as a commodity, of the commodity-capital, always consists of the wear and tear of the fixed capital, that is to say, of that portion of the value of the fixed capital which is transferred to the product during the process of production. The difference is really this: The fixed capital continues to function in the process of production in its old use-form for a longer or shorter cycle of turnover periods of the circulating capital (equal to constant circulating plus variable circulating capital), while every single turnover is conditioned on the replacement of the entire circulating capital passing from the sphere of production — in the form of commodity-capital — into the sphere of circulation. The constant circulating and variable circulating capital have the first phase of circulation, C' — M, in common. In the second phase they separate. The money into which the commodity is reconverted is in part transformed into a productive supply (constant circulating capital). Depending on the different terms of purchase of its constituent parts, one portion of the money may sooner, another later, be converted from money into materials of production, but finally it is wholly consumed that way. Another portion of the money realised by the sale of the commodity is held in the form of a money-supply, in order to be gradually expended in the payment of the labour-power incorporated in the process of production. This part constitutes the variable circulating capital. Nevertheless the entire replacement of either portion always originates from the turnover of capital, from its conversion into a product, from a product into a commodity, from a commodity into money. This is the reason why, in the preceding chapter, the turnover of the circulating capital, constant and variable, was treated jointly and separately without paying any regard to the fixed capital.