Mr. Downing - I do not understand how tannic acid can be a specific food for the strawberry. This has been asserted by Prof Mapes, at a meeting of the American Institute Farmers' Club, and your correspondent, Dr. Hull, and yourself, allude to it in the fore" of Prof. M.'s idea, unless the statement of his - put forth at the same meeting of the American Institute Farmers' Club. - that "toads eat strawberries," is to be taken as an illustration. Perhaps it is held that tannic acid is a specific food for toads, and lhat the toughness of their skins is owing to the tanning they have undergone from the effects of that acid, contained in the strawberries the animals had eaten!

But does any one suppose that tannic acid exists in the strawberry? If it did, it would be an anomaly, as the malic, tartaric, and citric, are the distinguishing acids of fruits; and though I cannot refer to any chemical examination of the strawberry, it is reasonable to suppose that its acid is one of these. Tannic acid may, to be sure, exist in the stein and root, and so it does in many other plants, to which it has never been thought of applying it as food. Plants have probably the power of elaborating their own acids.

Again, is it even known that old tan-bark contains any appreciable amount of tannic acid? It is well known that tanners use it as long as any tanning "liquor" can be made from it, and it is only thrown aside as "spent tan," when the tanning principle has been thoroughly extracted. The acid which then remains is probably the gallic. The beneficial effects of the bark as a mulch, are probably chiefly mechanical - preserving the moisture and friability of the soil. When mixed with the soil, however, it decomposes slowly, and may thus form a source of carbonic acid, on which it is known plants feed. But its value as manure is little or nothing till its natural acid is gone.

Can a single instance be cited of the beneficial application of tannic acid to plants? As a general thing, we know there is nothing more unwholesome for them than astringent acids. For example, peat or muck, from hemlock swamps, (and sometimes from other localities,) frequently contains a considerable amount of tannic acid, and when first dug, it is not only valueless as food for plants, but positively injurious - turning the leaves yellow, and checking the growth. The substance does not operate as manure, till rain, frost, and air have dissipated the acid.

It is true that strawberries have been known to grow well in reclaimed bogs; so have Indian corn, potatoes, beans, &c; but is there any more evidence that tannic acid contributed to the growth of the former than the latter? Neither will grow well, till the surface of the soil has become changed and sweetened by decomposing agents. Cranber ries, however, grow naturally, on bogs, without any preparation of the soil: but it has ever been pretended that tannic acid was a specific food for this plant, and its sour principle is known to be citric acid. H.

Atomy, Aug. 1,1851.