DEW, a dense, moist vapour, found on the earth in spring and summer mornings, in form of a misting rain, being collected there chiefly while the sun is below the horizon.
It hath been disputed whether the dew is formed from the vapours ascending from the earth during the night-time, or from the descent of such as have been already raised through the day. The most remarkable experiments adduced in favour of the first hypothesis are those of Mr Dufay of the Royal Academy of Sciences at Paris. He supposed, that if the dew ascended, it must wet a body placed low down sooner than one placed in a higher situation: and, if a number of bodies were placed in this manner, the lowermost would be wetted first; and the rest in like manner, gradually up to the top.
To determine this, he placed two ladders against one another, meeting at their tops, spreading wide afunder at the bottom, and so tall as to reach 32 feet high. To the several steps of these he fastened large squares of glass like the panes of windows, placing them in such a manner that they should not overshadow one another. On the trial it appeared exactly as Mr Dufay had apprehended. The lower surface of the lowest piece of glass was first wetted, then the upper, then the lower surface of the pane next above it; and so on, till all the pieces were wetted to the top. Hence it appeared plain to him, that the dew consisted of the vapours ascending from the earth during the night-time; which, being condensed by the coldness of the atmosphere, are prevented from being dissipated as in the day-time by the sun's heat. He afterwards tried a similar experiment with pieces of cloth instead of panes of glass, and the result was quite conformable to his expectations. He weighed all the pieces of cloth next morning, in order to know what quantity of water each had imbibed, and found those that had been placed lowermost considerably heavier than such as had been placed at the top; tho' he owns that this experiment did not succeed so perfectly as the former.
M. Muschenbroek, who embraced the contrary opinion, thought he had invalidated all Mr Dufay's proofs, by repeating his experiments, with the same success, on a plane covered with sheet-lead. But to this Mr Dufay replied, that there was no occasion for suppo-
ing the vapour to rise through the lead, nor from that very spot; but that as it arose from the adjoining open ground, the continual fluctuation of the air could not but spread it abroad, and carry it thither in its ascent.
But though this experiment of M. Muschenbroek's is not sufficient to overthrow those of Mr Dufay, it must still remain dubious whether the dew rises or falls. One thing which seems to favour the hypothesis of its descent is, that in cloudy weather there is little or no dew to be observed. From this M. de Luc brings an argument in favour of the hypothesis just now mentioned. He accounts for it in the following manner. Phil. Trans. vol. lxxii. part 2. When there were no clouds in the air, the heat of the inferior air and that which rises from the earth dissipates itself into the superior regions; and then the vapours which are dispersed throughout the air, condense, and fall down in dew: But, when the clouds continue, they separate the inferior from the superior part of the atmosphere, and thus prevent the dissipation of the heat, by which means the vapours remain suspended. When the sky grows cloudy, some hours after sunset, although the heat has been sensibly diminished, it is again increased; because, continuing to rise out of the earth, it is accumulated in the inferior air. But neither can this be reckoned a positive proof of the descent of the dew; since we may as well suppose the heat of the atmosphere to be great enough to dissipate it in its ascent, as to keep it suspended after its ascent through the day.
On the other hand, its being found in greater quantities on bodies placed low down than on such as are high up, is no proof of the ascent of the dew; because the same thing is observed of rain. A body placed low down receives more rain than one placed in an elevated situation; and yet the rain certainly descends from the atmosphere. The reason why the dew appears first on the lower parts of bodies may be, that, in the evening, the lower part of the atmosphere is first cooled, and consequently most disposed to part with its vapour. It is also certain, that part of the water contained in the air may be condensed at any time on the sides of a glass, by means of cold, so as to run down its sides in small drops like dew. It seems, therefore, that this subject is not sufficiently determined by such experiments as have yet been made; nor indeed does it appear easy to make such experiments as shall be perfectly decisive on the matter.
Several substances, exposed to the same dew, receive and charge themselves with it in a very different manner; some more, others less, and some even not at all. The drops seem to make a sort of choice of what bodies they shall affix themselves to: glass and crystals are those to which they adhere in the most ready manner, and in the largest quantity; but metals of all kinds never receive them at all, nor do the drops ever adhere to them. The reason of this is probably because metals promote evaporation more than glass does. Thus, if a piece of metal and a piece of glass are both made equally moist, the former will be found to dry in much less time than the latter. Hence it would seem, that there is between metals and water some kind of repulsion: and this may be sufficient to keep off the very small quantity that falls in dew; for whatever tends to make water evaporate after it is actually in contact with
Dew. with any substance, also tends to keep the water from ever coming into contact with it. On this subject several curious particulars are mentioned by Dr Percival, relative to the attraction and repulsion between dew and glass or metalline vessels. The experiments were made by M. du Fay, who, in order to determine with certainty whether the difference between vitrified substances and metals was the same in all cases, set a china faucer in the middle of a silver plate, and on one side, adjoining to it, was placed a china plate, with a silver dish very much resembling the faucer in the middle. In this experiment the china faucer was covered with dew, but the plate, though extending four inches round it, was not moistened in the least. The china plate also had become quite moist, while the silver vessel in the middle had not received the smallest drop. M. du Fay next endeavoured to ascertain whether a china faucer set upon a plate of metal, as already described, did not receive more dew than it would have done if exposed alone. To accomplish this design, he took two watch crystals of equal dimensions, and placed the one upon a plate of silver, the other upon a plate of china, each with its concavity uppermost. That which was upon the silver plate he surrounded with a ferrel of the same metal, well polished, that no watery particles might attach themselves to the convex surface of the glass. In this situation he exposed the crystals for several days successively, and always found five or six times more dew in that which was on the china plate than on the other placed on the silver. The repulsion between the dew and silver is further confirmed by the following experiment of M. du Fay, with regard to the crystal on the silver plate. He informs us, that the small quantity of dew on the inside near the centre, was in minute drops; and that round the border there was a space of five or six lines perfectly dry; towards which the drops regularly decreased in magnitude, as if the silver ferrel had driven away the dew from that part of the glass which was contiguous to it. These experiments were repeated thirty times with invariable success. M. du Fay's experiments have received a remarkable confirmation from some lately made by Dr Watson, now bishop of Landaff, with a view to determine the quantity of vapour that ascends from a given surface of earth. "By means of a little bees-wax (says he), I fastened a half-crown very near, but not quite contiguous, to the side of the glass; and, setting the glass with its mouth downward on the grass, it presently became covered with vapour, except that part of it which was next the half-crown. Not only the half-crown itself was free from vapour, but it had hindered any from settling on the glass which was near it; for there was a little ring of glass surrounding the half-crown, to the distance of a quarter of an inch, which was quite dry, as well as that part of the glass which was immediately under the half-crown; it seemed as if the silver had repelled the water to that distance. A large red wafer had the same effect as the half-crown; it was neither wetted itself, nor was the ring of glass contiguous to it wetted. A circle of white paper produced the same effect, so did several other substances, which it would be too tedious to enumerate."
SUBSTANCES of a very different kind from the usual dew are said to have sometimes fallen from the atmosphere. In the Phil. Trans. we are told, that in
the year 1695 there fell in Ireland, in the provinces of May-Dew Leinster and Munster, for a considerable part of the winter and spring, a fatty substance resembling butter, instead of the common dew. It was of a clammy texture, and dark yellow colour; and was, from its great resemblance, generally called dew-butter by the country people. It always fell in the night, and chiefly in the moorish low grounds; and was found hanging on the tops of the grass, and on the thatch of the houses of the poor people. It was seldom observed to fall twice in the same place; and usually, wherever it fell, it lay a fortnight upon the ground before it changed colour; but after that it gradually dried up, and became black. The cattle fed in the fields where it lay as well as in others, and received no harm by it. It fell in pieces of the bigness of one's finger-end; but they were dispersed scatteringly about, and it had an offensive smell like a church-yard. There were in the same places very stinking fogs during the winter, and some people supposed this no other than a sediment from the fog. It would not keep very long, but never bred worms.
May-Dew whitens linen and wax; the dew of autumn is converted into a white frost. Out of dew putrified by the sun, arise divers insects, which change apace from one species into another: what remains is converted into a fine white salt, with angles like those of salt-petre, after a number of evaporations, calcinations, and fixations.
There is a spirit drawn from May-dew, which has wonderful virtues attributed to it. The method of collecting and preparing it, is prescribed by Hanneman, physician at Kiel. It is to be gathered in clean linen cloths; exposed to the sun in close vials; then distilled, and the spirit thrown upon the caput mortuum; this is to be repeated till the earth unite with the spirit, and become liquid; which happens about the seventh or eighth cohobation or distillation. By such means you gain a very red, odoriferous spirit. Stolterfoht, a physician of Lubec, thinks May-dew may be gathered in glass-plates, especially in still weather, and before sun-rise. And Etmuller is of the same sentiment. It might likewise be collected with a glass funnel, exposed to the air, having a crooked neck to bring the dew into a vial in a chamber. See Phil. Trans. no 3. Hoffman, and others. It is apparently from the preparation of this dew, that the brothers of the Rosy-Crofs took their denomination. See ROSICRUCIANS.
Dew-Born, in country affairs, a distemper in cattle, being a swelling in the body, as much as the skin can hold, so that some beasts are in danger of bursting. This distemper proceeds from the greediness of a beast to feed, when put into a rank pasture: but commonly when the grass is full of water. In this case the beast should be stirred up and down, and made to purge well: but the proper cure is bleeding in the tail; then take a grated nutmeg, with an egg, and breaking the top of the shell, put out so much of the white as you may have room to slip the nutmeg into the shell; mix them together, and then let shell and all be put down the beast's throat; that done, walk him up and down, and he will soon mend.
Dew-Worm. See LUMBRICUS.
DE WIT (John), the famous pensionary, was born in 1625, at Dort; where he prosecuted his studies so diligently, that, at the age of 23, he published Ele-
De Wit. menta Curvarum Linearum, one of the deepest books in mathematics at that time. After taking his degrees, and travelling, he, in 1650, became pensionary of Dort, and distinguished himself very early in the management of public affairs. He opposed with all his power the war between the English and the Dutch; and when the event justified his predictions, he was unanimously chosen pensionary of Holland. In this capacity he laboured to procure a peace with Cromwell; in which peace a secret article was introduced by one side or other, for the exclusion of the house of Orange. In the war with England after the king's restoration, when it was thought expedient, on Opdam's defeat and death, that some of their own deputies should command the fleet, he was one of the three put in commission; and wrote an accurate relation of all that happened during the expedition he was engaged in, for which, at his return, he received the solemn thanks of the States-General. In 1667, he established the perpetual edict for abolishing the office of Stadtholder, to fix the liberty of the republic, as it was hoped, on a firm basis; which produced seditions and tumults, that restored the office, on pretence that the De Wits were enemies to the house of Orange, and plundered the state. The pensionary begged dismissal from his post; which was granted, with thanks for his faithful services. But the invasion of the French, and the internal divisions among the Hollanders themselves, spread every where terror and confusion; which the Orange party heightened to ruin the De Wits. Cornelius, the pensionary's brother, was imprisoned and condemned to exile; and a report being raised that he would be rescued, the mob armed, and surrounded the prison where the two brothers then were together, dragged them out, barbarously murdered them, hung the bodies on the gallows, and cut them to pieces, which many of them even broiled, and ate with savage fury. Such was the end of one of the greatest geniuses of his age; of whom Sir William Temple, who was well acquainted with him, writes with the greatest esteem and admiration. He observes, that when he was at the head of the government, he differed nothing in his manner of living from an ordinary citizen. His office, for the first ten years, brought him in little more than 300l. and in the latter part of his life, not above 700l. per annum. He refused a gift of 10,000l. from the States-General, because he thought it a bad precedent in the government. With great reason, therefore, Sir William Temple, speaking of his death, observes, "He was a person that deserved another fate, and a better return from his country; after 18 years spent in their ministry, without any care of his entertainments or ease, and little of his fortune. A man of unwearied industry, inflexible constancy, sound, clear, and deep understanding, and untainted integrity; so that whenever he was blinded, it was by the passion he had for that which he esteemed the good and interest of his state. This testimony is justly due to him from all that were well acquainted with him; and is the more willingly paid, since there can be as little interest to flatter, as honour to reproach, the dead."
Besides the works already mentioned, he wrote a book containing those maxims of government upon which he acted; which will be a never-fading monu-
ment to his immortal memory. A translation of it from the original Dutch, intitled, The true interest and political maxims of the republic of Holland, has been printed in London; to the last edition of which, in 1646, are prefixed historical memoirs of the illustrious brothers Cornelius and John de Witt, by John Campbell, Esq.