The light of the great orb of day emanates solely from a closely fitting robe of surpassing brightness. The great bulk of the sun which lies within that brilliant mantle is comparatively obscure, and might at first seem to play but an unimportant part so far as the dispensing of light and heat is concerned. It may indeed be likened to the coal-cellar from whence are drawn the supplies that produce the warmth and brightness of the domestic hearth; while the brilliant robe where the sun develops its heat corresponds to the grate in which the coal is consumed. With regard to the thickness of the robe, we might liken this brilliant exterior to the rind of an orange, while the gloomy interior regions would correspond to the edible portion of the fruit. Generally speaking, the rind of the orange is rather too coarse for the purpose of this illustration. It might be nearer the truth to affirm that the luminous part of the sun may be compared to the delicate filmy skin of the peach. There can be no doubt that if this glorious veil were unhappily stripped from the sun, the great luminary would forthwith lose its powers of shedding forth light and heat. The spots which we see so frequently to fleck the dazzling surface, are merely rents in the brilliant mantle through which we are permitted to obtain glimpses of the comparatively non-luminous interior. As the ability of the sun to warm and light this earth arises from the peculiar properties of the thin glowing shell which surrounds it, a problem of the greatest interest is presented in an inquiry as to the material composition of this particular layer of solar substance. We want, in fact, to ascertain what that special stuff can be which enables the sun to be so useful to us dwellers on the earth. This great problem has been solved, and the result is extremely interesting and instructive; it has been discovered that the material which confers on the sun its beneficent power is also a material which is found in the greatest abundance on the earth, where it fulfils purposes of the very highest importance. Let us see, in the first place, what is the most patent fact with regard to the structure of this solar mantle possessed of a glory so indescribable. It is perfectly plain that it is not composed of any continuous solid material. It has a granular character which is sometimes perceptible when viewed through a powerful telescope, but which can be seen more frequently and studied more satisfactorily on a photographic plate. These granules have an obvious resemblance to clouds; and clouds, indeed, we may call them. There is, however, a very wide difference between the solar clouds and those clouds which float in our own atmosphere. The clouds which we know so well are, of course, merely vast collections of globules of water suspended in the air. No doubt the mighty solar clouds do also consist of incalculable myriads of globules of some particular substance floating in the solar atmosphere.