ferment, in order to mix and assimilate its oily, watery, and saline parts. Swine's dung is of a saponaceous and oily nature, and the richest, perhaps, of all the oily manures. The dung of ruminant animals, as cows and sheep, is preferable to that of horses at grass, owing to the quantity of animal juices mixed with their food in chewing; and the fatter the animal, the richer is the dung. Human ordure, which is full of oil and a volatile alkaline salt, is of itself too strong a manure for any land. The dung of carnivorous animals isplentifully stored with oil; animals that feed upon seeds and grains come next; and after them follow those that subsist upon grass only.

The argument, moreover, in favour of oil being the principal food of plants, is confirmed by the observation, that, all vegetables, whose seeds are of an oily nature, are found to be remarkable impoverishers of the soil; as hemp, rape, and flax and the best manures for lands worn out by these crops are such as have a good deal of oil in their composition, provided that they are laid on with chalk, lime, marl, or soap-ashes, so as to render the oily particles miscible with water.

It must be further observed, that oily particles constitute the nourishment of plants in their embryo state; and, by a fair inference, we may suppose, that something of the same nature is continued to them as they advance in growth. The oily seeds, as rape, hemp, line, and turnip, consist of two lobes, which, when spread upon the surface, form the seminal leaves: in these the whole oil of the seed is contained. The moisture of the atmosphere penetrates the cuticle of the leaves, and mixing. with the oil, constitutes an emulsion for the nourishment of the plant. The oleaginous liquor being consumed, the seminal leaves, having performed their office, decay. The leguminous and

farinaceous plants keep their placenta, or seminal leaves, within the earth; in which situation they supply the tender germ with oily nutriment, until its roots are grown sufficiently strong to penetrate the soil.

It has been a received opinion, that lime enriches the land upon which it is laid, by supplying it with a salt fit for the nourishment of plants; but by all the experiments that have been made upon lime, it is found to contain no kind of salt. Its operation, therefore, should be considered different by the fermentation which it occasions, the earth is opened and divided; and, by its absorbent and alkaline quality, it unites the oily and watery parts of the soil. It seems also to have the property of collecting the acid of the air, which it readily forms into a neutral salt, of great use in vegetation. It is probable, therefore, that lime tends to rob the soil of its oily particles, and will, in time, render it barren, unless care be taken to support it with rotten dung, or other manures of an oily nature. Its great excellence, upon a sandy soil, is by mechanically binding the loose particles, and thereby preventing the liquid parts of the manure from escaping out of the reach of the radical fibres of the plants. Upon clay its effect is different; for, by the gentle fermentation it produces, the unsubdued soil is opened and divided the manures laid on readily come into contact with every part of it; and the fibres of the plants have full liberty to spread themselves. Clay, well limed, will fall in water, and ferment with acids; and, in these circumstances, the air, rains, and dews are freely admitted, and the soil is unable to retain the nourishment which they respectively afford. In consequence of a fermentation raised in the soil, the fixed air is set at liberty, which promotes vegetation in a surprising manner.

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I have already observed, in my former paper, that to this universal principle of oil, Dr. Hunter adds another, which is the nitrous acid of the air; for, although experiments have not yet confirmed the efficacy of nitre, considered as a manure, he regards it as a vivifying principle in the process of vegetation.

In order to conceive how plants receive their nourishment, we must recur to the analogy that is known to subsist between plants and animals. Oil and water seem to make up the nourishment of both. Earth enters very little into the compo.. sition of either. It is known that animals take in a great many earthy particles at the mouth, but they are soon discharged by urine and stool. Vegetables take in the smallest portion imaginable of earth; and the reason is, that they have no way to discharge it. It is highly probable, that the radical fibres of plants take up their nourishment from the earth, in the same manner that the lacteal vessels absorb the nutriment from the intestines; and as the oily and watery parts of our food are perfectly united into a milky liquor, by means of the spittle, pancreatic juice, and bile, before they enter the lacteals, we have all the reason imaginable to keep up the analogy, and suppose that the oleaginous and watery parts of the soil are also incorporated, previous to their being taken up by the absorbing vessels of the plant. To form a perfect judgment of this, we must recollect, that every soil, in a state of nature, has in itself a quantity of absorbent earth, sufficient to incorporate its inherent oil and water; but when we load it with fat manures, it becomes essentially necessary to bestow upon it, at the same time, something to assimilate the parts. Lime, soap-ashes, kelp, marl, and all the alkaline substances, perform that office. In order to

render this operation visible to the senses, dissolve one drachm of Russian potash in four ounces of water; then add one spoonful of oil; shake the mixture, and it will instantly become a uniform mass of a whitish colour, adapted to all the purposes of vegetation. This easy and familar experiment is a just representation of what happens after the operation of burn-baking, and, consequently, may be considered as a confirmation of the hypothesis advanced. In this process, the sward being reduced to ashes, a fixed alkaline salt is produced; the moisture of the atmosphere soon reduces that salt into a fluid state, which, mixing with the soil, brings about a union of the oily and watery parts, in the manner demonstrated in the experiment. When the under stratum consists of a rich vegetable mould, the effects of burn-baking will be lasting; but when the soil happens to be thin and poor, the first crop frequently suffers before it arrives at maturity. The farmer, therefore, who is at the expense of paring and burning a thin soil, should bestow upon it a portion of rotten dung, or shambles manure, before the ashes are spread, in order to supply the deficiency of oily particles: in this way, the crop will be supported during its growth, and the land will be preserved in health and vigour. Vegetables that have a succulent leaf, such as vetches, peas, beans, and buck-wheat, draw a great part of their nourishment from the air, and, on that account, impoverish the soil less than wheat, oats, barley,` or rye, the leaves of which are of a firmer texture. Rape and hemp are oil-bearing plants, and, consequently, impoverishers of the soil; but the former less so than the latter, on account of the great succulency of its leaf. The leaves of all kinds of grain are succulent for a time, during which period

the plants take little from the earth; but as soon as the ear begins to be formed, they lose their softness, and diminish in their attractive power. The radical fibres are then more vigorously employed in extracting the oily particles of the earth, for the nourishment of the seed. Dr. Hales observes, that the leaves of plants serve, not only as excretory ducts, to separate and carry off the redundant watery fluid, which, by being long detained in the plants, would turn tumid and prejudicial to them; but likewise to imbibe the dew and rain, which contain salt, sulphur, &c. and to be of the same use to plants that the lungs are to animals. He adds, however, that as plants have not a dilating and contracting thorax, their inspirations and expirations will not be so frequent as those of animals, but depend entirely on the alternate changes from hot to cold for inspiration, and vice versa for expiration. But the greater part of their nourishment is derived from their roots'. These, therefore, are found to bear a considerable proportion to the body of the plant above ground; the superficies of the former being above four tenths of that of the latter. Hence appears the necessity of cutting off many branches from a transplanted tree; because, in digging it up, a great part of the roots is cut off.

The facts which I have stated above, with regard to the influence of putrid air, arising from dunghills, &c. on the nourishment and increase of plants, were published by Dr. Hunter, in his Georgical Essays in 1769 and 1770; and they were afterward ascertained in the most satisfactory manner, by a variety of subsequent experiments by Dr. Priestley; which led that great philosopher

'See No. XXVIII. Reflections on Vegetation,