In capitalist society, however, the connections between these two great departments depend upon exchange of commodities, on the exchange of equivalents. The workers and capitalists of Department I can only obtain as many provisions from Department II as they can deliver of their own commodities, the means of production. The demand of Department II for means of production, on the other hand, is determined by the size of its constant capital. It follows therefore that the sum of the variable capital and of the surplus value in the production of producer goods [here I(_1,000v + 1,000s_)] must equal the constant capital in the production of provisions [here II(2,000_c_)]. An important proviso remains to be added to the above scheme. The constant capital which has been spent by the two departments is in reality only part of the constant capital used by society. This constant capital is divided into two parts; the first is fixed capital--premises, tools, labouring cattle--which functions in a number of periods of production, in every one of which, however, only part of its value is absorbed by the product, according to the amount of its wear and tear. The second is circulating capital such as raw materials, auxiliary semi-finished products, fuel and lighting--its whole value is completely absorbed by the new product in every period of production. Yet only that part of the means of production is relevant for reproduction which is actually absorbed by the production of value; without becoming less correct, an exact exposition of social circulation may disregard the remaining part of the fixed capital which has not been absorbed by the product, though it should not completely forget it. This is easy to prove.