Light and Air, then, being two good things, what happens when they come together? Spirit and water combined, says the toper, are two good things spoiled; and how do light and air mix? Pick out of Cheapside the busiest of men, and he will tell you that he loves the sky-blue in its proper place, making a sickly joke about his milk-jug. There is not a Scrub in the whole world who would not think it necessary to show pleasure--yes, and feel some indication of it--over sunset colors, when, by chance, he treads the fields upon a summer evening. We all look up at the stars, and feel that they would seem much less the confidential friends they really are, if they were shining down upon us with a rigid light. There is a beating human pulse which answers to our hearts in their incessant twinkling. And then the rainbow! Light that might pass down to us, and give us sight, but nothing more, gives sight and blesses it at once. Its touch converts the air into a region of delightful visions, ever changing, ever new. To reach us it must penetrate our atmosphere, and it is a fact that He who made the Universe, so made it that, in the whole range of Nature there is not one barren combination. Light must pass through the air; and, from a knowledge of the other laws of Nature, it might confidently be proclaimed, that, in addition to the useful purposes of each, and their most necessary action on each other, beauty and pleasure would be generated also by their union, to delight the creatures of this world. It is not our design just now to talk about the nature of the atmosphere; to attempt any analysis of light, or even to mention its recondite mysteries. But in a plain way we propose to look into the reason of those changes made by light in the appearance of the sky, those every-day sights with which we are the most familiar.