In the second place, it is not only the new value added in the
 process of production by the week’s labour which enters completely into
 the weekly product, but also the value of the raw and auxiliary materials
 consumed by the weekly product. This value circulates with the product
 containing it. It assumes the form of money through the sale of the product
 and must be reconverted into the same elements of production. This applies
 as much to the labour-power as to the raw and auxiliary materials. But
 we have already seen (Chapter VI, II, 1) that continuity of production
 requires a supply of means of production different for different branches
 of industry, and different within one and the same branch of business for
 different component parts of this element of the circulating capital, for
 instance, for coal and cotton. Hence, although these materials must be
 continually replaced in kind, they need not always be bought anew. The
 frequency of purchases depends on the size of the available stock, on the
 time it takes to exhaust it. In the case of labour-power there is no such
 storing of a supply. The reconversion into money of the part of capital
 laid out in labour-power goes hand in hand with that of the capital invested
 in raw and auxiliary materials. But the reconversion of the money, on the
 one hand into labour-power, on the other into raw materials, proceeds separately
 on account of the special terms of purchase and payment of these two constituents,
 one of them being bought as a productive supply for long periods, the other,
 labour-power, for shorter periods, for instance a week. On the other hand
 the capitalist must keep a stock of finished commodities besides a stock
 of materials for production. Let us leave sales difficulties aside. A certain
 quantity of goods must be produced, say, on order. While the last portion
 of this lot is being produced, the finished products are waiting in the
 warehouse until the order can be completely filled. Other differences in
 the turnover of circulating capital arise whenever some of its separate
 elements must stay in some preliminary stage of the process of production
 (drying of wood, etc.) longer than others.