Investigation of the circulation of money in society shows that the individual capitalist can never invest the whole of his money capital in production but must always keep a certain money reserve to be employed as variable capital, i.e. as wages. Further, he must keep a capital reserve for the purchase of means of production at any given period, and in addition, he must have a cash reserve for his personal consumption. The process of reproducing the total social capital thus entails the necessity of producing and reproducing the substance of money. Money is also capital, for Marx's diagram which we have discussed before, conceives of no other than capitalist production. Thus the diagram seems incomplete. We ought to add a further department, that of production of the means of exchange, to the other two large departments of social production [those of means of production (I) and of consumer goods (II)]. It is, indeed, a characteristic feature of this third department that it serves neither the purposes of production nor those of consumption, merely representing social labour in an undifferentiated commodity that cannot be used. Though money and its production, like the exchange and production of commodities, are much older than the capitalist mode of production, it was only the latter which made the circulation of money a general form of social circulation, and thus the essential element of the social reproductive process. We can only obtain a comprehensive diagram of the essential points of capitalist production if we demonstrate the original relationship between the production and reproduction of money and the two other departments of social production. Here, however, we deviate from Marx. He included the production of gold (we have reduced the total production of money to the production of gold for the sake of simplicity) in the first department of social production. 'The production of gold, like that of metals generally, belongs to department I, which occupies itself with means of production.'[92] This is correct only in so far as the production of gold is the production of metal for industrial purposes (jewellery, dental stoppings, etc.). But gold in its capacity as money is not a metal but rather an embodiment of social labour _in abstracto_. Thus it is no more a means of production than it is a consumer good. Besides, a mere glance at the diagram of reproduction itself shows what inconsistencies must result from confusing means of exchange with means of production. If we add a diagrammatic representation of the annual production of gold as the substance of money to the two departments of social production, we get the following three sets of figures: I. _4,000c + 1,000v + 1,000s = 6,000_ means of production II. _2,000c + 500v + 500s = 3,000_ means of subsistence III. _20c + 5v + 5s = 30_ means of exchange