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taken place to a considerable depth. Mr. Hunt's experiments prove that the electro-negative metals make the most decided images upon electro-negative plates, and vice versa. M. Matteucci has shown that a discharge of electricity does not visibly affect a polished silver plate, but that it produces an alteration which renders it capable of condensing vapour.

M. Fizeau ascribes a numerous class of these phenomena to the action of a slight layer of organic or fatty matter on the surfaces, which, being volatile, is transferred to any body near, in a greater or less quantity according to the distance; that is, according as the surface projects or sinks into hollows. When the different parts of a surface are unequally soiled by extraneous bodies, even in the minutest quantity, the condensation of mercurial vapour is effected in a manner visibly different on its different parts, and therefore images are formed. Although this explains various phenomena, it does not apply to those already described, as Mr. Hunt had taken the precaution to divest the substances he used of every trace of organic matter.

It is difficult to see to what cause Mr. Hunt's experiments on the reciprocal action of bodies in total darkness can be attributed, unless, perhaps, to a constant radiation of some peculiar principle from their surfaces, which really seems to exist.

The impression of an engraving was made by laying it face downwards on a silver plate iodized, and placing an amalgamated copper plate upon it: it was left in darkness fifteen hours, when an impression of the engraving had been made on the amalgamated plate, through the paper.

As the same may be obtained on plates of iron, zinc, or lead, it is evident that this result is not the effect of chemical rays.

An iodized silver plate was placed in darkness with a coil of string laid on it, and with a polished silver plate suspended one-eighth of an inch above it; after four hours they were

exposed to the vapours of mercury, which became uniformly deposited on the iodized plate, but on the silver one there was a sharp image of the string, so that this image was formed in the dark, and even without contact. Coins or other objects leave their impressions in the same manner with perfect sharpness and accuracy, when brought out by vapour without contact, in darkness, and on simple metals.

Heat, electricity, and the evaporation of unctuous matter, may account for some of these phenomena, but others clearly point at some unknown influence exerted between the surfaces of solid bodies, and affecting their molecular structure so as to determine the precipitation of vapours, an influence which in all probability will ultimately be found to be either the parathermic rays of Sir John Herschel, or intimately connected with them.

SECTION XXVI.

Atmosphere of the Planets and the Moon-Constitution of the Sun-Estimation of the Sun's Light-His influence on the different Planets-Temperature of Space-Internal Heat of the Earth-Zone of Constant Temperature -Heat increases with the depth-Heat in Mines and Wells-Thermal Springs-Central Heat-Volcanic Action-The Heat above the Zone of Constant Temperature entirely from the Sun-The Quantity of Heat annually received from the Sun-Isogeothermal Lines-Distribution of Heat on the Earth-Climate-Line of Perpetual Congelation-Causes affecting Climate -Isothermal Lines-Excessive Climates-The same Quantity of Heat annually received and radiated by the Earth.

THE Ocean of light and heat perpetually flowing from the sun must affect the bodies of the system very differently, on account of the varieties in their atmospheres, some of which appear to be very extensive and dense. According to the observations of Schröeter, the atmosphere of Ceres is more than 668 miles high, and that of Pallas has an elevation of 465 miles. These must refract the light and prevent the radiation of heat like our own. But it is remarkable that not a trace of atmosphere can be perceived in Vesta. The action of the sun's rays must be very different on such bodies from what it is on the earth, and the heat imparted to them quickly lost by radiation; yet it is impossible to estimate their temperature, since the cold may be counteracted by their central heat, if, as there is reason to presume, they have originally been in a state of fusion, possibly of vapour. The attraction of the earth has probably deprived the moon of hers; for the refractive power of the air at the surface of the earth is at least a thousand times as great as refraction at the surface of the The lunar atmosphere, therefore, must be of a

moon.

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greater degree of rarity than can be produced by our best air-pumps; consequently no terrestrial animal could exist in it. This was confirmed by M. Arago's observations during the last great solar eclipse, when no trace of a lunar atmosphere was to be seen.

The sun has a very dense atmosphere, which is probably the cause of the peculiar phenomena in his photographic image already mentioned. What his body may be, it is impossible to conjecture; but he seems to be surrounded by a mottled ocean of flame, through which his dark nucleus appears like black spots often of enormous size. These spots are almost always comprised within a zone of the sun's surface, whose breadth, measured on a solar meridian, does not extend beyond 3040 on each side of his equator, though they have been seen at the distance of 3910. From their extensive and rapid changes, there is every reason to suppose that the exterior and incandescent part of the sun is gaseous. The solar rays, probably arising from chemical processes that continually take place at his surface, or from electricity, are transmitted through space in all directions; but, notwithstanding the sun's magnitude, and the inconceivable heat that must exist at his surface, as the intensity both of his light and heat diminishes as the square of the distance increases, his kindly influence can hardly be felt at the boundaries of our system, or, at all events, it must be but feeble.

The direct light of the sun has been estimated to be equal to that of 5563 wax candles of moderate size, supposed to be placed at the distance of one foot from the object. That of the moon is probably only equal to the light of one candle at the distance of twelve feet. Consequently, the light of the sun is more than three hundred thousand times greater than that of the moon. Hence the light of the moon imparts no heat. Professor Forbes is convinced, by recent experiments, that the direct light of the moon is incapable of raising a thermometer one three-hundred-thousandth part of a centi

grade degree, at least in this climate. The intensity of the sun's light diminishes from the centre to the circumference of the solar disc.

In Uranus, the sun must be seen like a small but brilliant star, not above the hundred and fiftieth part so bright as he appears to us; but that is 2000 times brighter than our moon; so that he is really a sun to Uranus, and may impart some degree of warmth. But, if we consider that water would not remain fluid in any part of Mars, even at his equator, and that, in the temperate zones of the same planet, even alcohol and quicksilver would freeze, we may form some idea of the cold that must reign in Uranus.

The climate of Venus more nearly resembles that of the earth, though, excepting perhaps at her poles, much too hot for animal and vegetable life as they exist here; but, in Mercury, the mean heat arising only from the intensity of the sun's rays must be above that of boiling quicksilver, and water would boil even at his poles. Thus the planets, though kindred with the earth in motion and structure, are totally unfit for the habitation of such a being as man, unless, indeed, their temperature should be modified by circumstances of which we are not aware, and which may increase or diminish the sensible heat so as to render them habitable.

It is found, by experience, that heat is developed in opaque and translucent substances by their absorption of solar light, but that the sun's rays do not sensibly alter the temperature of perfectly transparent bodies through which they pass. As the temperature of the pellucid planetary space can be but little affected by the passage of the sun's light and heat, neither can it be sensibly raised by the heat now radiated from the earth; consequently its temperature must be invariable, at least throughout the extent of the solar system. The atmosphere, on the contrary, gradually increasing in density towards the surface of the earth, becomes less pellucid, and therefore gradually increases in temperature, both from the direct action of the sun, and from

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