Pine Bluff daily graphic., 1894-09-26
The Work of Dust.

when we look up into the sky we see the dust in the atmosphere illumin- ated 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 can be no blue Armament 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 back ground, and the same sharp con trast would prevail upon the illumin- ated surface of the earth-bllnding light where tho suns rays fall and deep black shadows where they do not Only the light of the moon and stars, which would remain visible in the daytime, would be able to tenn per this contrast in q slight degree
 The illumination of the earth 'ssur- face would be like that we see with the telescope on the lunar landscape; for he moon has no atmospheric envelope that can hold floating dust
 We hen owe 10 dust the even, mod erately y-tempered daylight, adapted now to our eyes; and it is that which contributes much to the beauty of our landscape scenery.
 But if dust makes the sky appear clear, why is the color of the sky blue? Why does dust, of the different constituents of white sunlight, re fleet the niue rather than the green, yellow and red? This fact is con nected with the size of the dust par tieles Only the finest dust settles SD slowly that it can be spread every where by means of thc air currents, and cah bp feHnd constantly ill all strata Of the atmosphere; HnG special importance can be ascribed only to taese finest particles. The coarse parts soon iall to the ground. Let us consider the fine mechanism Of light, the extremely short ether waves which determine its eXistance. These waves. although they are of even less than microscopic size, are not at all equally long. The short est are those that give blue light, while all the other colors are pro duced bylOnger waves. The fine at mosphere dust contains many par ticles which are large enough to re- fleet the short blue ether waves fewer than can reflect green and vel low, and still fewer large enough to reflect the long red waves. The red light, therefore, goes on almost with out hindrance, while the blue is more liable to be diverted, and thus to reach the eye. X similar phenome- non may be observed on a larger scale on water which is roughened with waves of different lengths, and on which pieces of wood are floating
 The pieces of wood stand in the same relation to the water waves as the dust particles to the ether waves. The great long waves pass the blocks undisturbed, only rocking up and down; while the finer ripples of the water are turned back, as if the blocks were firm walls.
 The finest dust thus appears blue. There is much coarse dust in large towns, when the sky over them is often gray, while only the finest blue dust is carried up in thecountry. The dust is also variously assorted at different heights above the surface of the earth. The cOarser dust will be found at the lower levels
 where 1t is produced. On mountains we have most of the dust beneath us, while the rarefied air can sustain only the finest floating particles Hence the sky on high mountains is clear and deep blue, even almost black, as if it were without dust Only when we look at the lower strata, toward the horizon, does the color pass int o gray.
 Why is the sky in Italy and the tropics of a so much deeper blue than that of Western Europe? Is the dust there finer? It is really so; not that a finer quality of dust is produced there, but because in the moist climate of the North Sea countries the dust cannot float long in the air without being charged with water and made coarser, while in warmer countries water exists in the air as yapor and does not become con densed as liquid on the dust Only when it is carried by the air currents into the higher etrata and is cooled there, does it thicken into clouds With this we come to the most i'm portant function of dust in our at mosphere-the part which it has in the function of rain, by reason oi