it hath been disputed whether the dew is formed from 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 favor of the first hypothesis are thouse of Dr. 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 the 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 to ladders against one another, meeting at their tops, spreading wide asunder 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 overshade one another. On the trial it appeared exactly as Dr 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 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 quantidy of water each had imbibed, and found thouse that had been placed lowermost considerably heavier than such as had been placed at the top; though he owns that this experiment did not succeed so perfectly as the former.
M. Muchenbroek, who embraced the contrary opinion, thought he had invalidated all Dr. Dufay`s proofs, by repeating his experiments, with the same success, on a plane covered with sheet-lead. But to this Dr. Dufay replied, that there was no occasion for supposing the vapour to rise through the lead, nor from that very spot; but that as it arose from the adjoining open ground, the continual fluctation 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 thouse of Dr 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 it cloudy weather there is still 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: When there are 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 from inferior from the superior part of the atmosphere, and thus prevent the dissipation of the heat, by which means the vapors remain suspended.
When the sky grows cloudy, some hours after sunset, altough 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 supposed 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 of bodies placed low down than on such as are high up, is no proof of 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 vapours. It is also certain, that part of the water contained in the air may be condensed at any time of 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 deciseve on the matter.
Several substances exposed to the same dew receive and change 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 thouse to which they adhere in the most ready manner, and in the largest quantidy; 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. This, if a piece of metal and a piece of glass are both made qually 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 of some kind of repulsion; and this may be sufficient to keep off the very small quantity of falls in dew; from whatever tends to make water evaporate after it is actually in contact 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. Dufay, who, in order to determine with certainty whether the difference between vitrified substances and metals was the same in all cases, set a china saucer 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 saucer in the middle.
In this experiment the china saucer was covered with dew, but the plate, though extending for inches round it, was not moistened in the least. The china plate also had some quite most, while the silver vessel in the middle had not received the smallest drop.
M. Dufay next endevoured to ascertain whether a china saucer 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 qual 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. Dufay, with regard to the crystal on the silver plate. He informs us, that the small quantidy of dew on the side 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. Dufay`s experiments received a remarkable confirmation, from some lately made by Dr Watson, now bishop of Llandaff, with a view to determine the quantity of vapor 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 which was next to 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 Leiuster and Munster, for a considerable part of the winter and spring, a fatty substance resembling butter, instead of 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 color; 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 churchyard. 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 diverse insects, which change apace from one species into another; what remain is converted into a fine white salt, with angles like thouse of saltpetre, 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 described 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 eight 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. Number 4. Hoffman and others.
It is apparently from the preparation of this dew, that the brothers of the Rosy-Cross took their domination.
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