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Since the Contemplative Philosopher was originally written, a variety of theories of dew have been advanced by different persons. One of the most ingenious is that stated by Dr. Wells in his Essay on Dew,' published in 1814. Mr. Wilson and Mr. Six thought the formation of dew was accompanied by the evolution of cold; and this opinion was once held by Dr. Wells. But subsequent observations led him to question its accuracy; and he was not long after enabled to ascertain, by direct experiments, that the temperature of bodies sinks before any dew is deposited on them; and that the subsequent deposition of dew is the consequence of this coldness. This philosopher infers, therefore, that the deposition of dew has precisely the same cause as the appearance of moisture on the outside of a glass, or metallic vessel, when a liquor considerably colder than the air has been shortly before poured into it.
All bodies have the property of radiating heat. During the day, the heat lost by radiation is more than supplied by the solar heat; so that the temperature of bodies is increased during the day, instead of being diminished. But during the night the heat radiated by the bodies on the surface of the earth penetrates into the sky and does not again return to them. Hence their temperature must be constantly diminishing from radiation, and they will become and continue colder than the air during the whole night; thus being in the state for the deposition of dew upon their surfaces. This, however, will only happen when the sky is clear, and the atmosphere calm. If the sky be covered with clouds, they will radiate back nearly as much heat as they receive; and thus prevent the terrestrial bodies from cooling considerably. And in windy nights, the agitation of the atmosphere compensates for its bad conducting power, and thus prevents that
rapid lowering of temperature requisite to the production of dew.
Upon these and analogous principles Dr. Wells accounts for the various phenomena of dew, as well as several other appearances which he attributes to similar causes: for these, however, the inquisitive reader must turn to the Essay itself.
ON THE FOOD OF PLANTS.
Each tree, each plant, from all its branching roots,
IN treating, in a former paper', on the influence of the air on vegetation, I was led to take notice of the pabulum, or food of plants, as a subject of future discussion. This is a subject, indeed, which has been productive of considerable dispute. What is generally understood by the term itself, is such matter, of whatever kind, as being added and united to the first stamina of plants, at their sowing, or to their roots, and thence to their trunks and other parts afterward, gives them their increase, or, in other words, is itself the matter of that increase. The great art of the husbandman is the giving this food to plants,
1 No. XV.
in the best manner, and to the greatest advantage. But, before he can expect to arrive at any true knowledge in this article, it is necessary that he should know what this food or matter of nourishment is. There are five things generally allowed to contribute to the growth and increase of plants; namely, nitre, water, air, fire, and earth. But it has been much disputed which of all these it is that increases, or is properly the food of the plant. Much has been said of the acid spirit residing in the air; but as this is sharp enough to corrode iron bars, it is too sharp to be fit for nourishing tender plants. Nitre is said to nourish them; but its true office is not actually nourishing, but preparing other things to nourish them. Nitre applied to the root of a plant will kill it; but, at a distance, it so attenuates, cuts, and divides the viscous matter found in the earth, that after this they are fit to nourish plants, although they were not so before. Water has been thought by lord Bacon, and others, to be the only food of plants; but a famous experiment of Van Helmont, usually brought to prove this, is nothing to the purpose, for the water contains earth, and, therefore, is never given to a plant as water alone.
Air, on account of its elasticity, is necessary to the increase of plants; but it does not follow, that it is the matter of that increase, although Bradley and others have taken great pains to prove that it is so. The experiments of Dr. Priestley, indeed, render it a natural conclusion, as mentioned in my paper above referred to, that phlogiston is the principal food of plants, upon which their nourishment and increase very much depend. As to fire, we are very well assured, that no plant can live without heat; but, although different degrees of heat are necessary to different plants, yet none can live in actual fire; nor has fire been ever
thought the food of plants, except by Mr. Lawrence, and some of the metaphysicians. Others maintain, that earth alone is the true food and matter of increase in plants. Every plant, say the advocates for this opinion, is earth; and the growth and increase of that plant is only the addition of more earth to it, in the same form. They add, that nitre, and other salts, as those of dung, &c. prepare, attenuate, and divide the earth, destined for the increase of vegetables: that water and air move it, by conveying, and fermenting it in the juices; but that earth itself is still the food. When the additional earth is once assimilated to the plant, it becomes, they assert, a part of it, and remains for ever with it; but let water, air, and heat be taken away, and the plant remains a plant still, although a dead one. They add further, that the excess of the other things is a proof, that they are not the proper food of the plant, by destroying, instead of nourishing it: for too much nitre, or other salts, corrodes and kills the plant; too much water drowns it; too much air dries the roots of it; too much heat burns it; but too much earth a plant can never have, unless entirely buried under it, so as to exclude the ne cessary action of the rest of the assistant articles.
But how ingenious soever these arguments may be, more modern experiments and observations have led the way to another theory with respect to the food of plants, which seems to be less exceptionable than either of the theories I have stated. The sentiments of the illustrious Dr. Hales I have already mentioned in my former paper; in which I likewise intimated, what I shall now proceed to explain more particularly, the principle to which Dr. Hunter has had recourse. This gentleman, so justly celebrated, both as an anatomist and a natural philosopher, was induced
by a number of experiments to conclude, that all vegetables receive their principal nourishment from oily particles incorporated with water, by means of an alkaline salt, or absorbent earth. Till oil is made miscible, it is unable to enter the radical vessels of vegetables; and, on that account, Providence has bountifully supplied all natural soils with chalky or other absorbent particles; and those soils which have been assisted by art are full of materials for that purpose; such as lime, marl, soap-ashes, and the volatile alkaline salt of putrid dunghills. Natural soils receive their oily particles from the air or atmosphere, which, during the summer months, is full of putrid exhalations from the steam of dunghills, the perspiration of animals, and smoke; and every shower brings down these oleaginous particles for the nourishment of plants. All rich soils, in a state of nature, contain oil; and, in lands that have been ploughed for some years, it is found in proportion to the quantity of putrid dung that is laid upon them, allowance being made for the crop they have sustained. By attending to the effects of manures of an oily nature, Dr. Hunter apprehends that we must be satisfied, that oil, however modified, is one of the chief instruments of vegetation. Thus rape-dust, laid on land, is a certain and speedy manure; as it contains the food of plants ready prepared. Soot is also an oily manure, but different from the former, because it contains alkaline salt, in its own nature calculated as well for opening the soil, as for rendering the oily parts miscible with water. The dung of pigeons, which feed chiefly upon grains and oily seeds, is a rich and quick manure. The dung of horses that are kept in stables, and fed upon beans, oats, and hay, which contain much oil, is also a strong manure, more especially when it has undergone the putrid