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does not form equally on all substances, because different substances radiate heat with various degrees of force, and a scale may be formed of these substances, showing their radiating powers—as, say, Wool, Cotton, Down, Grass, Leaves, Sand, Glass, Porcelain, Varnish, Wood, and Metals.
From this scale it appears that the best radiating bodies are organic and silicious, and this is more particularly the case when they present large surfaces, from which the heat can pass freely. Any thing that tends to compress or condense the substance into a more compact body, injures its radiating power. Thus a loose fleece of wool, if compressed into a comparatively solid mass, will not radiate equally well; whilst polishing a piece of metal will deprive it of a portion of its radiating power.
Why there should be this difference in the radiating powers of substances, we do not know. There seems to be some relation between the conducting and radiating properties of various bodies, the best conductors being the worst radiators. The extent of surface also appears to have considerable effect, as the greater the surface the greater the radiation; hence the large amount of radiation from leaves of trees and grass.
Counter radiation has, however, wherever it takes place, its full degree of effect in producing the general result. The under sides of the upper fibres of loose wool and leaves of trees, like a cloud above the earth, will radiate heat1 downwards, and to a proportionate extent counteract upward radiation from lower objects. And any contiguous lateral substance will have a similar effect, as it will reciprocate the radiation. A covering of the slightest kind may thus counterbalance the upward radiation.
Radiation of heat is, then, the cause of that cooling of the surface of the earth during the night which takes place under a clear sky, and that produces liquid dew on the earth: but the cooling thus produced is not equally great in all parts under apparently similar circumstances. It is the greatest in the interior of large continents, and more particularly where there is a very dry atmosphere, as in parts of Russia, the Desert of Bokhara, the great Desert of Northern Africa, and other similar parts. Accounts of travellers in such places represent the cold produced in them by radiation during clear nights, as being more intense than in other countries having a damper atmosphere. The cause of this difference is, however, not to be sought for in any greater clearness of the atmosphere in dry than in damp climates, as the fact is rather the reverse of this, the air being somewhat clearer in the damp countries; but in a process which, when dew is formed, always counteracts to a certain extent the cooling effect of radiation.
When radiation cools the surface of the earth so much as to condense and liquefy some of the aqueous vapour that is in the air, that liquefaction liberates much heat: and this heat tends to warm the part that is in course of being cooled by radiation. There is, then, a double process going on at the same time and in the same place. Radiation is cooling the part, whilst liquefaction of vapour is warming it; and, under these circumstances, it is only to the extent that the influence of the former exceeds the latter, that cooling is accomplished. When there is much vapour in the atmosphere, much of it is soon liquefied, and the cooling effect of radiation is thereby counteracted to a great extent; when there is little vapour, there is less liquefaction of that vapour, and cooling is consequently less counteracted. And where the atmosphere is so dry as not to admit the liquefaction of any vapour from the degree of cold that exists, radiation produces its effect without being in any degree counteracted by recently liberated heat.
In such dry deserts as those referred to, the cold in the early part of the night, when it produces dew, produces it only on the best radiators, which are generally the few vegetables that are found in the deserts; and, as the cold increases, worse radiators have dew deposited on them successively in the order of their radiating powers.
In our own country, from the operation of the cause here pointed out, radiation does not produce that intense cold in the early part of the winter, when the dew-point is comparatively high, that it does at a later period of the season, when the dew-point is very low. In the latter part of the winter, as there is not sufficient vapour to permit much of it to be liquefied by the cold of radiation, that cold may, and frequently does, go on increasing without counteraction during the absence of the sun.
Thus we find that vapour, when condensed into liquid by cold, always gives out heat; whether it is in the formation of the cumulous cloud in the higher regions of the atmosphere, in producing mist near the surface of the earth, or in the production of dewdrops on the surface, the same effect is experienced; and, wherever heat is liberated, it must have its degree of influence in counteracting the cold of radiation.
It has been stated, that "metals give to glass near which they are placed, the property of more speedily attracting caloric from hot air; and, on the contrary, that of yielding it more speedily to cold air," because a mercurial thermometer accommodates itself to a higher temperature sooner than an air thermometer. But this may be because the heat which passes into the glass tube of the thermometer is rapidly absorbed by the mercury. In like manner, when placed in a colder medium, the heat of the mercury is conducted to the inner surface of the glass tube more rapidly than is the heat of the inclosed air; the mercury therefore cools quicker than the air. But these results are consequences of mercury being a better conductor of heat than air is. And when a piece of foil is placed on the inside of a pane of glass, the outside of the glass opposite the foil is not so soon cooled by radiation, because tlie metal furnishes heat to supply the place of that lost by radiation from the glass.
We may then say that "falling dew" is produced by the descent of the cumulus or day-cloud, which, cooled by evaporation in a higher part of the atmosphere, sinks in the evening to the surface of the earth. Floating dew is found in parts which have much vapour in the air in proportion to the temperature, along with a clear atmosphere, and when, consequently, radiation from the surface cools the atmosphere contiguous to it, and condenses a portion of the vapour which the air contains into minute globules of liquid, which are sustained by the elastic force of the air; whilst dew, properly so called—that which is found attached to various substances in the form of drops—is a result of the cooling of certain bodies below the dew-point of the atmosphere by radiation of heat from those bodies, and a consequent condensation and abstraction of some of the vapour which the air resting on them contained. And the more any body is thus cooled, the greater will be the quantity of dew deposited on it. In the two last-mentioned modes, dew supplies to a certain extent the place of rain. Where clouds are freely formed, rain falls on the earth to some extent; but when rain is absent, and the sky is cloudless, radiation of heat, by cooling and condensing vapour, gives some moisture to the earth. Thus the sands of Africa and Asia, which are never visited by rain, have their scanty vegetation supplied with a certain amount of that moisture which is so essential to the life of organized beings.
On the means of computing the quantity of Aqueous Vapour in a Vertical Column of the Atmosphere.
If there were an empty atmospheric space over the surface of our globe, the vapour which would arise from water by the process of evaporation, would, by its elastic force, expand upwards, and constitute an atmosphere of pure vapour, of a height and density that would be determined by the temperature. As explained by Dalton, the density would be the greatest in the lowest part, as the portion in that part would sustain the whole incumbent mass, and would be less in higher parts, until at a great height the density would be inconsiderable, the vapour being greatly rarefied by its own expansive force. The temperature would not be the same at the different heights, but, in accordance with the known laws of cooling by expansion of all aeriform substances, it would diminish as the vapour expanded.
In such an atmosphere the temperature and density at different heights may be presumed to be as expressed in the following table, No. 1, where the temperature and dew-point at the surface is assumed to be 50°, and the quantity of vapour at certain heights would be as great as could exist in the aeriform state at each height.