Soil B yields for the first invested capital of £2½ — 2 quarters per acre, and for the second investment of equal magnitude — 1½ quarters; together — 3½ quarters from the same acre. It is not possible to distinguish which part of these 3½ quarters is a product of invested capital I and which part a product of invested capital II, for it is all grown upon the same soil. In fact, the 3½ quarters is the yield from the total capital of £5; and the actual fact of the matter is simply this: a capital of £2½ yielded 2 quarters, and a capital of £5 yielded 3½ quarters rather than 4 quarters. The situation would be just the same if the £5 yielded 4 quarters, i.e., if the yield from both investments of capital were equal; similarly, if the yield were even 5 quarters, i.e., if the second investment of capital were to yield a surplus of 1 quarter. The price of production of the first 2 quarters is £1½ per quarter, and that of the second 1½ quarters is £2 per quarter. Consequently the 3½ quarters together cost £6. This is the individual price of production of the total product, and, on the average, amounts to £1 14 2/7 sh. per quarter, i.e., approximately £1¾. With the general price of production determined by soil A, namely £3, this results in a surplus-profit of £1¼ per quarter, and thus for the 3½ quarters, a total of £4 3/8. At the average price of production of B this corresponds to about 1½ quarters. In other words, the surplus-profit from B is represented by an aliquot portion of the output from B, i.e., by the 1½ quarters, which express the rent in terms of grain, and which sell — in accordance with the general price of production — for £4½. But on the other hand, the excess product from an acre of B over that from an acre of A does not automatically represent surplus-profit, and thereby surplus-product. According to our assumption, an acre of B yields 3½ quarters, whereas an acre of A yields only 1 quarter. Excess product from B is, therefore, 2½ quarters but the surplus-product is only 1½ quarters; for the capital invested in B is twice that invested in A, and thus its price of production is double. If an investment of £5 were also to take place in A, and the rate of productivity were to remain the same, then the output would be 2 quarters instead of 1 quarter, and it would then be seen that the actual surplus-product is determined by comparing 3½ with 2, not 3½ with 1; i.e., it is only 1½ quarters, not 2½ quarters. Furthermore, if a third investment of capital, amounting to £2½, were made in B, and this were to yield only 1 quarter — this quarter would then cost £3 as in A — its selling price of £3 would only cover the price of production, would provide only the average profit, but no surplus-profit, and would thus yield nothing that could be transformed into rent. The comparison of the output per acre from any given soil type with the output per acre from soil A does not show whether it is the output from an equal or from a larger investment of capital, nor whether the additional output only covers the price of production or is due to greater productivity of the additional capital.