thermometer with a blackened bulb, the different radiating powers of the various coatings will be immediately

detected.

The greatest heat will be found radiating from the lamp-black, the least from the polished surface, and if, for the sake of illustration, the heat from the lamp-black be supposed equal to one hundred, that from the writing paper is ninety-eight, from the jelly eighty, and from the polished surface only twelve.

“During all these experiments, it will be remarked how singularly these effects of radiation are opposed to the conducting powers of the respective surfaces; if we touch the clean part of the canister, it burns us; but we may place the finger with impunity upon the lamp-black, paper, or the jelly, which, though good radiators, are bad conductors of heat."

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Radiation not only proceeds from the immediate particles which form the surface of a body, but likewise from those at some distance beneath, and this curious fact may be easily determined; for example, take a cylindrical tin canister, and cover one half of its surface with a coating of extremely thin jelly, and the other half with four or five coatings of the same, letting one coat dry before another is applied; when all are dry, fill the canister with boiling water.

In this arrangement, although the nature of the two surfaces is precisely the same as regards material and smoothness, they will radiate very differently; the hand held at some little distance from the thin coating will not feel so hot as when held at the same distance from the thick coating, and if a thermometer, with a blackened bulb, be similarly held, it will indicate a small

elevation of temperature in the one case, but great in the other.

"In this experiment the increase of radiating power must be attributed to the increased quantity of radiating material, and it is discovered to continue until the coating amounts to the thickness of about one-hundredth part of any inch; after which no further increase takes place; it is inferred, therefore, that the particles constituting the jelly have the power of radiating, from a thickness below the actual surface, equal to the above amount.'

As regards terrestrial radiant heat, meaning by the term that of a fire, or any other artificial source, the chemist discovers that surfaces, if similar in mechanical texture, may differ in color, without having their receptive or radiating power either increased or diminished; whilst, on the other hand, with solar radiant heat, he discovers that the colors of such surfaces do certainly affect both these powers most strangely; for dark-colored, or black, surfaces absorb and radiate more rapidly than such as are light-colored, or white.

Paint the bulb of one thermometer with the black mixture already mentioned, and the bulb of another, similar in size and graduation, with a mixture of whitening and glue, of equal thickness with the black; let these bulbs dry, and then expose the thermometers simultaneously to bright sunshine it will be immediately found that the black bulb absorbs heat quicker than the white bulb, by the greater rise of the mercury in its tube; and upon removing the two thermometers into the shade, probably the black bulb will indicate that it radiates heat quicker than the white bulb, by the more rapid descent of the mercury in its tube.

Again, nine pieces of equally fine kerseymere cloth, seven of them having the prismatic colors, and of the others, one black, and the other white, upon exposure to sunshine for the same time, will heat very differently, and of the colors, probably in the following order, the first being the hottest; violet, indigo, blue, green, red, orange and yellow; but the black cloth will surpass even the violet in temperature, whilst the white cloth will not even attain that of the yellow, as the application of the bulb of a thermometer will immediately prove.

From an experiment of this kind, we discover that "the warmth or coolness of clothing depends as well on its color as its quality; a white dress, or one of a light color, will always be cooler than one of the same quality, black, or of a dark color, and especially so in clear weather, where there is much sunshine; white, or presence of color, and very light hues, reflect heat copiously; but black, or absence of color, and very dark hues, reflect little."

"Experience has long supplied the place of science in directing the choice of clothing; the use of light colors always prevails in summer, and that of dark colors in winter."

Reasoning upon this property of black and dark surfaces in absorbing heat, a natural conclusion is drawn that the black skin 66 or rete mucosum" of the Ethiopian, would cause him to undergo severe suffering under the intensity of the tropical solar rays; but our experiments, be it remembered, have been made either upon inorganic matters, or matters deprived of vitality, and they do not hold good with bodies under the influence of the recon

dite agencies of life, an extraordinary fact or anomaly, so called by the finite wisdom of man, is then presented.

The Creator, in His power and goodness, has mercifully ordained, that the skin of the Ethiopian shall absorb heat powerfully, but, at the same time, that it shall not sustain the slightest injury.

This extraordinary fact was originally investigated by a physiologist, in a series of extremely simple yet decisive experiments, of which the following are selected:

During an intensely hot day, he exposed the back of each hand to the sunshine for ten minutes, one hand being bare, and having a thermometer attached to it, the other being covered with black cloth, and having the bulb of a thermometer beneath this.

At the expiration of the above time, the exposed thermometer indicated eighty-five degrees, whilst the covered thermometer indicated ninety-one degrees; in a second trial, the former indicated ninety-one degrees, the latter ninety-four degrees; and in a third trial, the first thermometer indicated ninety-eight degrees, whilst the second indicated one hundred and six degrees.

In these three trials, the bare skin was powerfully scorched and raised in blisters, but the covered skin, though so much hotter, was not in the slightest degree injured.

Upon covering one hand with white cloth, and the other with black cloth, and exposing them for a similar time to sunshine, the former was invariably scorched, but the latter never affected.

The power of the solar rays to scorch the skin of man and animals is destroyed when these surfaces are black, although the absolute heat in consequence of the absorption of the rays is greater.

"The same wise Providence which has given so extraordinary a provision to the Negro for the defence of his skin, while living within the tropics, has extended it to the bottom of the eye, which otherwise would suffer in a greater or less degree when exposed to strong light; the retina from its transparency allowing it to pass through without injury."

The black or dark matter of the "retina" is technically called the "nigrum pigmentum," and is probably not absolutely necessary for vision, but only provided as a defence against strong light, since it is found to be much darker in the Ethiopian than in the European, and is of a lighter color in fair people than in dark, and therefore lightest in those countries farthest removed from the effects of the sun.

The scorching power of the solar rays being destroyed when received by black surfaces is an incontrovertible fact, and the only explanation that the chemist can offer regarding it is, that the mixture of heat and light in the solar rays is absorbed by black surfaces, and converted into sensible heat.

The sudden and fatal infliction called "the sunstroke" that frequently seizes reapers and laborers, in the intense heat of harvest time, is referred to the scorching effect of the solar rays, upon the scalp of the head, producing inflammation of the brain, and probably the first record of this is in the following Scripture passage.

"And when the child was grown, it fell on a day that he went out to his father to the reapers. And he said unto his father, My head, my head! And he said to a lad, Carry him to his mother. And when he had taken him, and brought him to his mother, he sat on her knees till noon, and then died."