[There are few more beautiful processes in nature than the formation of dew, and few which are so generally misunderstood - "The falling dew" being in fact only a piece of pure poetry. The following interesting explanation by M. Melloni, a European savan of distinction, is the latest and best that we have seen. It was originally published in the Compte Rendus, in a more elaborate form, but has been condensed and translated by Dr. Lindley, in the following letters. Ed.]

First Letter

From the experiments undertaken by Wells to explain the true cause of dew, it seems quite clear, I think, that dew neither rises from the earth nor falls from the sky formed by the elastic and invisible vapor which is present everywhere in the atmosphere; the precipitation of aqueous vapor is clearly owing to the cold produced by the radiation of bodies towards a clear sky. Looking at the question in this way, leaves, wood, glass, Varnish, lampblack, become covered with dew, because they emit heat easily, and are considerably cooled under a clear sky. Metals, on the other hand, remain dry, in consequence of the difficulty they have in radiating their heat to the upper regions of the atmosphere; and, in fact, a great difference is observed between the indications of a thermo-scope, when a vessel of polished metal, full of boiling water, and an exactly similar one coated with lampblack, are successively presented to it; the action of the second being much greater than that of the first. The deduction is correct; but it must be allowed that it does not necessarily appear so to everybody.

Indeed, Prevost, and Saussure before him, attributed the absence of dew on metals to an electric force; Leslie explained the same phenomenon by a particular repulsion, which, he said, existed between metallic surfaces and watery vapor; and those who maintained that dew arose from the earth, explained the same thing by the heat and electricity disengaged by the chemical action of metals upon the particles of this same vapor, at the moment of their passage to the liquid state. To show that these hypothesis are untenable, I first take three thermometers with graduated stems, and on each tube I fix a small cork about five or six millimetres above the bulb. This cork helps to support the metallic cases in which the thermometers for experiments on nocturnal cooling, are inclosed. The first case consists of a small, thin, polished silver or copper cup, like a common thimble, and large enough to contain the bulb of the thermometer; the second is a tin cylinder, open at one end and closed at the other; this serves as an envelope for the graduated tube.

The two metallic pieces (which can easily be put off or on,) are kept in their places by the elasticity of the cork.

In the next place I procured three tin cups, each having a lateral opening near its bottom, through which the bulbs of the prepared thermometers can be passed, while the stems with their envelopes remain horizontally on the outside. These cups are supported by fine metallic tubes, provided with covers of the same nature, and the whole were exposed to the air on a calm fine night. One of the thermometer cases was blackened, and the other two were in their natural state, and the cups were sometimes open and sometimes shut. Such was the apparatus with which I compared the nocturnal radiation of silver with that of lampblack. Suppose the cups to be first shut, the three thermometers then mark the same temperature. Then by opening two of the cups, and leaving the third, containing one of the bright thermometers, shut, it will be seen that the metallic thermometer which is now exposed to the air, fails so little that hardly any change can be observed, except with the finest instruments; while the thermometer coated with lampblack, falls very visibly, and after a few minutes it will mark three or four degrees less than the thermometer in the closed cup - an evident proof that this difference is owing to the heat radiated by the lampblack, and not at all to the contact of the exterior air, which equally surrounded the polished metallic casing of the other exposed thermometer.

My results confirm, in a striking manner, the assertion of MM. La Provostaye and Desains, viz: that the emissive power of metals is much less than the experiments of Leslie, Du-long and Petit, led people to suppose.

The radiating power of lampblack being 100, that of laminated silver I found to be 3.02G. MM. La Provostaye and Desains find 5.37 for silver chemically precipitated on copper, and 2.1 when the silver is polished. According to the last mentioned gentleman, the emissive power of recently laminated silver is 2.94; while 2.38 is that of lamiFrom observations made in 1838,I am led to believe that the differences in the radiating power of the polished and the scratched side of the cube in Leslie's famous experiment, is not owing, as was generally thought, to the differences in the mechanical state of the two surfaces, but to a change of density resulting from the scratching. This appears, moreover, to be confirmed by the three following facts: 1st. The variation in the emissive power is only observable in metals; marble, jet, or ivory have the same radiating power whether they are smooth or not. 2d. If silver be melted and slowly cooled in sand molds, then burnished and afterwards scratched with a diamond, so that the bottom of the scratches be compressed and condensed, its radiating power will be less after the scratching than it was before. 3d. This same piece of silver, melted and polished, has its radiating power diminished by being hammered or laminated.

Thermoscopic instruments similar to those described above, and having the cases covered with varnish, black lead, isinglass, sawdust, sand, dust and leaves, constantly indicated a very sensible fall of temperature before becoming moist with dew; the lapse of time between the fall of temperature and the deposit of dew, sometimes amounted to several hours; a fall of temperature, moreover, often occurred without any deposit of dew at all during the night. This last phenomenon occurred the more frequently the higher the thermometers were placed above the ground. By making your observations at a certain height above the soil, you can delay or entirely prevent the deposition of dew on your instruments, and prove conclusively that it always follows and never precedes the production of cold. . I have never seen the polished metallic cases of my thermometers covered with condensed vapor during damp nights, provided there was no trace of fog in the atmosphere.