Page:Popular Science Monthly Volume 45.djvu/667

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HEN a beam of sunlight enters a darkened room through a hole in the window shutter, it can be seen along its whole course. The light is reflected to every side, and made to reach the eye by the dust in the air of the room. We do not see the sunbeam itself, but the dust which is illuminated by it; and individual bodies can be perceived on a closer inspection floating in the beam. The dust may be much more plainly observed in still air, as it settles on objects. It is extremely slow in falling to the ground, although it consists of matter which in larger masses falls very speedily. This we can test by collecting dust and compressing it into a ball. In this process of compression a very large part of the exposed surface which the particles presented to the air is caused to disappear; and it was by means of this great extent of surface that the air bore enough upon the particles to support them against falling. The finer the dust the more extended is its surface in proportion to its mass, and the more it is delayed in falling through the air. It may seem useless to speak of the part played by this dust in Nature; for what noticeable effect can this insignificant stuff bring about? We have, however, as can be shown, no right to regard it as a little thing.

Dust has a very large share in nearly all the phenomena of the earth's atmosphere. It is what makes the clear sky appear blue; and when we look up into the sky we see the dust in the atmosphere illuminated by the sun. There is nothing else before us that can permit the light to reach the eye. Light goes invisible, straight through all gases, whatever their chemical composition. The dust catches it, reflects it in every direction, and so causes the whole atmosphere to appear clear, in the same way that it makes the sunbeam visible in the darkened room. Without dust there would be no blue firmament. The sky would be as dark as or darker than we see it in the finest moonless nights. The glowing disk of the sun would stand immediately upon this dark background, and the same sharp contrast would prevail upon the illuminated surface of the earth—blinding light, where the sun's rays fall, and deep black shadows where they do not. Only the light of the moon and the stars, which would remain visible in the daytime, would be able to temper this contrast in a slight degree. The illumination of the earth's surface would be like that we see with the telescope on the lunar landscapes; for the moon has no atmospheric envelope that can hold floating dust. We then owe to dust the even moderately tempered