Encyclopedia of Extract on the Dew - Diderot and d'Alembert
DEW, sf (Physiq.) aqueous meteor which can be distinguished into three species, namely the dew which rises from the earth into the air, the dew which falls from the air, & finally the dew that we see in the form of drops on the leaves of trees & plants. Let's run through these three species. 1°.
The dew rises from the earth by the action of the sun, during the summer months; the sun does not produce these effects at first sight, but imperceptibly, for as soon as it appears above the horizon, it begins to warm the earth & darts its rays there, & its heat continues to penetrate more deeply , until one or two hours after bedtime; that's when the heat starts to stop,
We can collect the dew, by putting in the evening on the ground, or a little above, unpolished metal plates, or large discs of glass. If, after a very hot day, these plates are placed in a place which has been well lighted by the sun, the vapor which rises from the earth will be carried against the lower surface and will attach itself to it, and if they are placed a little obliquely on the ground, the dew will flow towards the lower end, leaving behind it the traces which mark the route which it has taken; if, on the contrary, the plates are placed in a place which has not been illuminated by the sun, or which has been only very slightly, only a small quantity of dew will collect there.
When one is in the country, and when after a hot day, one comes to have a cold evening, one sees coming out of the canals and the ditches the vapor of the water, which rises like smoke; this vapor is no sooner a foot or two above the place from which it originates than it spreads equally on all sides; then the countryside soon appears covered with a dew which rises imperceptibly; it moistens all the bodies on which it falls, and wets the clothes of those who walk there.
The rising dew cannot be the same in the different parts of the earth. Indeed the dew will be found almost entirely composed of water in aqueous countries, near lakes and rivers, or in the neighborhood of the sea; but if the earth is greasy, sulphurous, full of wood, animals, fish, sown fields, the dew will then be composed of various kinds of oils, volatile salts, and the subtle spirits of plants; if the ground contains many minerals, the dew will also be composed of similar parts, as M. Boërhaave observes in his chymie. There is also a lot of dew in humid and watery countries, and less in dry and arid places, which are far from the sea, rivers or lakes; let us add that the dew does not always rise to the same height; the greater part stops very low, another part rises in the atmosphere, up to an average height, and the lesser part at a great height.
The dew having risen to a certain height, floats slowly in the air; sometimes it rises, sometimes it descends, surrounding all the bodies it finds to meet it, and sometimes it falls from the air to moisten the earth. Philosophers do not agree on this, but M. Musschenbroeck has made various experiments in this respect, which do not allow us to doubt the fall of the dew; they can be read in his physics essay, §. 1535. He made almost all these experiments on the observatory of Leyden, at the top of which there is a large platform, where he arranged in all directions pieces of stuff, tons, vases, bells, &c. who all received dew from the air.
The dew does not fall indiscriminately on all kinds of bodies; this assertion seems singular, and the skilful physicist whom we have just quoted, noticed that the different colors attract the dew with an unequal force; the inequality of their attractive force depends on the structure and size of the colored bodies.
There is no dew when there is a strong wind, because everything that rises from the ground is first carried away by the wind, and everything that has risen in the air during the day, is also stopped & carried away by the wind. Here are some observations of Mr. Musschenbroeck on this subject. "What are the winds with which the dew falls, or what are the winds which precede during the day, the fall of the evening dew? I have often been surprised to see dew falling with a north wind, because this wind being cold in this country, condenses the earth, & closes its openings; however, it does not fall so often, when this wind blows, as when there are other hot winds, so that one never picks up so much dew, that when the wind is south, south - west, & South East ; this is also what was observed formerly in Greece; for we learn from Aristotle that dew fell there with a south-east wind; it is not difficult to explain this phenomenon; the wind is hot, it opens the earth, it heats the vapors which then rise in great quantity, & can consequently fall in abundance, &c. " Loc. cit. §. 1538.
A lot of dew falls in the month of May, because the sun then sets in motion a large quantity of the earth's juices, and causes a lot of vapors to rise. The dew of May is more watery than that of summer, because the great heat volatilizes not only the water, but also the oils and the salts.
Aristotle, Pliny, & others, believed that the dew fell at night, because the stars & the moon pressed it down; and this is why the philosophers who came later added that the dew fell in great abundance when the moon was full, and that it shone all night. They called the moon, the mother of the dew, (Virg. georg. l. III.) & the dew, the daughter of the air & of the moon. (Plut. symp. 3.) However, we pick up just as much dew, and with the same ease, on nights when the moon does not shine, as in the light of this star; and what virtue could the rays of light that issue from it have, since if we receive them on the largest burning mirror, and by gathering them in the hearth, we condense them there five hundred times more, they do not produce the slightest effect on the most mobile thermometer. See HEAT, MOON, &c.
One can distinguish dew from rain; 1°. because the rain is white & clear water, whereas the dew is yellow & cloudy; 2°. in that pure distilled rainwater has neither smell nor taste, whereas distilled dew has both.
The third species of dew of which we have to speak, bears this name improperly; it is about those aqueous drops that one sees at daybreak on the leaves of plants and trees, after a dry night. It was believed that this liquor fell from the air, on the plants and on the grass, where it is found in such large quantities, that one could not cross a meadow in the morning without having wet feet. We are greatly mistaken in this respect, because the dew of plants is properly their sweat, and consequently a humor which belongs to them, and which issues from their excretory vessels.
Sometimes we see these drops gathered near the stem where the leaf begins, as in cabbage and poppies; at other times they are found on the outline of the leaves and on all the eminences, as can be seen, especially in watercress; sometimes they are seen in the middle of the sheet near the coast; they are also quite often found on the top of the leaf, as in meadow grass, &c. The origin of this dew can be explained thus, according to M. Musschenbroeck. When the sun heats the earth during the day, and sets in motion the humidity that is there, it rises and insinuates itself into the roots of the plants against which it is carried; after this moisture has once penetrated into the root, it continues to ascend higher, passing through the stem into the leaves, whence it is conducted by the excretory vessels, to the surface, where it collects in great quantity, while the rest remains in the plant; but this humidity first dries up during the day by the heat of the air, so that during the day we do not see any of it on the leaves, and since only a little liquor then returns to the stem & towards root, all plants seem to wither somewhat towards midday; the liquors that have been heated continue to move in the earth during the night, they come to go as during the day against the roots of the plants, they enter there just as before, & then rise upwards; but the plants are then all surrounded by colder air, which dries the fluids less, thus the juices which flow from the excretory vessels, and which do not dry up after leaving them, collect imperceptibly, and take on the form of drops, which are in the morning in all their size, unless they are dissipated by the wind, or dried up by the heat of the rising sun.
As this feeling is new, the same physicist, whom we have quoted throughout this article, endeavored to prove it by various very exact experiments, which he reports. 1533. of his physics essay.
The dew is healthy or harmful to animals and plants, depending on whether it is composed of round or sharp parts, sweet or acrid, saline or acid, spirituous or oleaginous, corrosive or terrestrial; that is why doctors attribute various diseases to dew. Vossius, according to Thomas Cantipratensis, in his book on bees, warns shepherds not to graze their flocks early in the morning in fields which are covered with dew, because the dew, which is extremely subtle, will insinuates into the viscera, that it sets the stomach in motion by its heat, and that it purges it with such violence that death sometimes ensues. Pliny's opinion, book. XVIII. vs. xxix. does not seem well-founded; he wants to prevent the dew from being harmful to the sown lands, we set fire to the wood, the straw and the herbs of the countryside or the vines, because this smoke will prevent all the evil that could happen; but this smoke could not produce any good effect, if it is not in the places where there are vapors and acid exhalations, which are then tempered by what there is of alkali in the smoke. It is said that oilseed dew is very unhealthy, especially for cattle, and it has been observed that the year is very sterile when a lot of this dew falls. It is claimed that in a certain year, the walnut trees died of it in Dauphiné, and that the leaves of the other plants were burnt, as well as the wheat and the vine; but we must attribute this malignity less to the dew than to the excessive heat of the sun. This article is by M. Formey, who took it from Essays in Physics by M. Musschenbroeck, already quoted several times in this article.
DEW, (Chemistry & Medicine.) Chemists have long supposed & sought in dew marvelous principles, precious emanations from all the kingdoms of nature, & panspermia of the atmosphere (see PANSPERMIE), which they have growths eminently proper to open up certain bodies, to alter them in various ways, to impregnate them, to enrich them with new qualities, &c. It is with these views that chemists have collected it with care, and sometimes even under mysterious circumstances; that they digested it, distilled it, fermented it, &c. & that they then employed it in various extractions, tinctures, &c. that they exposed various bodies to its influence, &c. It is from there that came to pharmaceutical chemistry the method of preparing the saffron of Mars to the dew,
The action of dew, well evaluated in its various operations and in its uses for some arts, as for the bleaching of linen and that of wax, has evidently proved to modern chemists that dew operates in all these cases only. as water; and that all the differences that could be observed between the effects of common water and those of dew, were very well explained by the various forms of application, namely in that common water was usually used in the form of a considerable mass or volume, remaining for a long time on the bodies to which it was applied, and that the dew was only applied to these bodies in the form of drops, of disintegrated molecules, or at most of a very thin layer. light, & easily dissipated,
The dew & the serene which is a species that has been characterized by imaginary differences (see SEREIN), considered as a non-natural thing, that is to say as an external object, exerting an influence on the animal body, still act only as water or as humidity, at most as cold humidity.
Dew must be counted among the external objects whose effects are most harmful to weak bodies and not accustomed to its action. Those who are subject to colds, coughs, chest diseases, ophthalmia, pains in the limbs, and colic, must above all very carefully avoid exposing themselves to it. (b)
DEW, (Sacred Criticism) ros; this word, besides the proper meaning, is taken in Scripture for manna; in the morning there fell a dew, ros, all around the camp, Exod. xvi. 13. It was the very manna that was collected around the camp. See MANNA.
As Palestine was a very hot country, and the dew was abundant there, this word also sometimes designates abundance, the quantity of something; hence this comparison; such as the cloud of dew, such is the day of abundant harvest, Isaiah xviij. 4. And elsewhere we will overwhelm it with our numbers, as when the dew falls on the earth. II. Kings, xviii. 12. (DJ)
ROSEE, farriers call the blood that begins to appear on the ground when it is trimmed to soothe the horse. See PARER & DESSOLER.
DEW OF THE SUN, (Botan.) Tournefort established in this genus of plant seventeen species, of which he names the principal, ros solis folio oblongo, in English, the common round-leav'd sundew.
Its root is fibrous & loose like hair. It sprouts several long, slender, hairy stems above, to which are attached small, almost round, concave ear-pick leaves, pale green, furnished with a fringe of fistulous reddish hairs, from which squirt a few droplets of liquor into cavities of leaves ; so that these leaves and their hairs are always wet with a kind of dew.
From between these leaves rise two or three stems almost half a foot high, slender, round, reddish, tender, devoid of leaves; they carry at their top small flowers with several petals, arranged in pink, whitish, leaning on the same side, supported by chalices formed in horn, jagged, & attached to very short pedicles. When these flowers are spent, they are succeeded by small fruits which have about the size and shape of a grain of wheat, and which contain several oblong or round seeds.
This plant flowers in June & July, & comes in deserted & sandy, harsh, humid places, & most often between mosses; it is viscous to the touch, so that on touching it its sticky liquor is drawn out like little whitish silky filaments, which take on a certain consistency at the time. This plant is considered pectoral, soothing, & good in the inveterate dry cough. (DJ)