Caroline. And, pray, why is the sky of a blue colour?
Mrs. B. You should rather say, the atmosphere; for the sky is a very vague term, the meaning of which, it would be difficult to define, philosophically.
Caroline. But the colour of the atmosphere should be white, since all the rays traverse it, in their passage to the earth.
Mrs. B. Do not forget that the direct rays of light which pass from the sun to the earth, do not meet our eyes, excepting when we are looking at that luminary, and thus intercept them; in which case, you know, that the sun appears white. The atmosphere is a transparent medium, through which the sun's rays pass freely to the earth; but the particles of which it is composed, also reflect the rays of light, and it appears that they possess the property of reflecting the blue rays, the most copiously: the light, therefore, which is reflected back into the atmosphere, from the surface of the earth, falls upon these particles of air, and the blue rays are returned by reflection: this reflection is performed in every possible direction; so that whenever we look at the atmosphere, some of these rays fall upon our eyes; hence we see the air of a blue colour. If the atmosphere did not reflect any rays, though the objects, on the surface of the earth, would be illuminated, the sky would appear perfectly black.
Caroline. Oh, how melancholy would that be; and how pernicious to the sight, to be constantly viewing bright objects against a black sky. But what is the reason that bodies often change their colour; as leaves, which wither in autumn, or a spot of ink, which produces an iron-mould on linen?
Mrs. B. It arises from some chemical change, which takes place in the arrangement of the component parts; by which they lose their tendency to reflect certain colours, and acquire the power of reflecting others. A withered leaf thus no linger reflects the blue rays; it appears, therefore, yellow, or has a slight tendency to reflect several rays, which produce a dingy brown colour.
An ink spot on linen, at first absorbs all the rays; but, from the action of soap, or of some other agent, it undergoes a chemical change, and the spot partially regains its tendency to reflect colours, but with a preference to reflect the yellow rays, and such is the colour of the iron-mould.
Emily. Bodies, then, far from being of the colour which they appear to possess, are of that colour to which they have the greatest aversion, with which they will not incorporate, but reject, and drive from them.
Mrs. B. It certainly is so; though I scarcely dare venture to advance such an opinion, whilst Caroline is contemplating her beautiful rose.