Ricardo says that rent can never, not even in the slightest degree, constitute an element in the price of corn. This is certainly not a very happy way of expressing the truth, that a high rent is not the cause, but the effect, of a relatively high price of corn.[153-1] Ricardo would have been nearer right had he said that rent was not a component part of the price of every portion of the supply of corn brought to market. Is rent an addition to national income? Ricardo (ch. 31) answers this question in the negative, and says that it takes from the consumers what it gives to the owners of the land, and that it increases only the value in exchange of the national wealth.[153-2] It is evident that as thus stated, the question is not properly put. Neither interest on capital nor wages are any addition to a nation's income, but, like rent, only forms of trade, by means of which that income is distributed among the individuals constituting the nation. (§ 201.) The special kind of product obtained from a piece of land influences its rent only in so far as the growth of that kind of product is exclusively confined either by nature, privilege or prejudice to certain land.[153-3] Adam Smith is of opinion that the rent of agricultural land is ordinarily (!) one-third of the gross product; that of coal mines, from one tenth to a maximum of one-fifth; of good lead and tin mines, one sixth (with the dues paid the state of twenty-one and two-thirds per cent.); of Peruvian silver mines, scarcely one-tenth; of gold mines, one-twentieth. And he thinks that rent grows less certain for every succeeding article.[153-4] So far as this is based on facts, it may be explained as follows: The greater capacity an article has for transportation from one place to another, the less important is advantage of situation, which is generally one of the chief elements of rent. The more indispensable the commodity is, the more readily is the consumer induced to pay a price for it greater than the cost of production; that is, to pay a rent. This again is enhanced by the difficulty of the preservation of the commodity. Lastly, the more it is a mere product of nature,[153-5] the more difficult it is to simultaneously employ several portions of capital of different grades of productiveness in its production.