such prices, as afford equal expectation of advantage to all classes of producers; which can only be when things [pg 373] exchange for one another in the ratio of their cost of production.
It sometimes happens [however] that two different commodities have what may be termed a joint cost of production. They are both products of the same operation, or set of operations, and the outlay is incurred for the sake of both together, not part for one and part for the other. The same outlay would have to be incurred for either of the two, if the other were not wanted or used at all. There are not a few instances of commodities thus associated in their production. For example, coke and coal-gas are both produced from the same material, and by the same operation. In a more partial sense, mutton and wool are an example; beef, hides, and tallow; calves and dairy produce; chickens and eggs. Cost of production can have nothing to do with deciding the value of the associated commodities relatively to each other. It only decides their joint value. Cost of production does not determine their prices, but the sum of their prices. A principle is wanting to apportion the expenses of production between the two.
Since cost of production here fails us, we must revert to a law of value anterior to cost of production, and more fundamental, the law of demand and supply. The law is, that the demand for a commodity varies with its value, and that the value adjusts itself so that the demand shall be equal to the supply. This supplies the principle of repartition which we are in quest of.
Suppose that a certain quantity of gas is produced and sold at a certain price, and that the residuum of coke is offered at a price which, together with that of the gas, repays the expenses with the ordinary rate of profit. Suppose, too, that, at the price put upon the gas