Now we have settled our preliminaries, we have got our principles; the next thing is to put them into practice. Let us first note what has been said of the absorption of light by transparent bodies. The air is one of the most transparent bodies known. On a clear day--when vapor (that is not air) does not mingle with our atmosphere--mechanical obstacles and the earth's figure form the only limits to our vision. You may see Cologne Cathedral from a mountain distant nearly sixty miles. Nevertheless, if the atmosphere had no absorbing power, only direct rays of the sun, or rays reflected from the substances about us, would be visible; the sky would be black, not blue; and sunset would abruptly pitch us into perfect night. The air, however, absorbs light, which becomes intermixed with its whole substance. Hold up your head, open your eyes widely, and stare at the noonday sun. You will soon shut your eyes and turn your head away; look at him in the evening or in the morning, and he will not blind you. Why? Remembering the earth to be a globe surrounded by an atmosphere, you will perceive that the sun's rays at noonday have to penetrate the simple thickness of the atmosphere, measured in a straight line upward from the earth; but in the evening or morning its beams fall aslant, and have to slip through a great deal of air before they reach us; suffering, therefore, a great deal of robbery; that is to say, having much light absorbed. Now, why is the sky blue? Not only does the air absorb light; it reflects it also. The particles of air reflect, however, most especially the blue ray, while they let the red and his companions slip by. This constant reflection of the blue ray causes the whole air to appear blue; but what else does it cause? Let us consider. If air reflects or turns aside, or hustles out of its place the blue ray, suffering the rest to pass, it follows as a consequence that the more air a ray of light encounters, the more blue will it lose. The sun's rays in the morning and the evening falling aslant, as we have said, across a great breadth of our atmosphere, must lose their blue light to a terrible extent, and very likely reach us with the blue all gone, and red lord paramount. But so, in truth, the case is; and the same fact which explains the blueness of the atmosphere, explains the redness of the sunrise and the sunset. It will now easily be understood, also, why the blue color of the sky is deepest in the zenith, faintest when we look over the horizon; why the blue is at noon deeper than after mid-day; why it grows more intense as we ascend to higher elevations. From what we have already said, the reason of these things will come out with a very little thought. Again, in the example of our London fogs, &c., when in the upper portion of the dense mass the blue rays have been all refracted, there can penetrate only those other rays which make the lurid sky, with which we are familiar, or the genuine old yellow fog.