Dust
fine earth capable of being carried in the wind. The smaller a particle, the more surface it presents to the wind in proportion to its weight. Doubling the diameter of a grain of dust multiplies its weight by the cube of two, or eight, and its surface by the square of two, or four. Trebling the diameter multiplies the weight by the cube of three, or twenty-seven, and the surface by the square of three, or nine, etc. The most prevalent dust is fine sand. Dust is carried several miles high into the air. In fact the atmosphere is never entirely free from dust, though heavy rains tend to bring it down. Grassy prairies and forests are almost free from serious dust; but in dry weather sandy tracts and large areas, in which fall plowing is the rule, are afflicted by dust storms disagreeable to the farmer and housekeeper alike. In the Sahara and other desert regions sand storms arise. Bird and beast seek shelter; the caravan halts; the Arab enfolds his face and person in a shawl; the faithful camel kneels down flat and stretches its long neck forward on the sand, closes its long, silky eyelashes, and narrows the slit of its nose. Even the tireless vulture lights and disposes its head and plumage till the suffocating cloud is by.
Though apt to be regarded as a nuisance, dust is, after all, an advantage to the world. Water does not soak through genuine dust, because the particles do not lie closely enough together to permit the action of so-called capillarity. Compact soil, whether clay, sand, or mold, will draw water or absorb it while dust remains dry. A dusty surface is a preventive against the loss of water by evaporation, and saves large areas from becoming deserts. In semi-arid or dry regions, where rain is not to be expected during the summer, farmers pulverize the surface of the soil with fine-toothed harrows and form a dust blanket as early in the spring as possible, to prevent the escape of moisture by evaporation. The solid earth beneath holds the moisture of winter and spring until it is drawn upon as needed by the deep roots of the growing crop. Luxuriant crops protected by dust blankets may be seen growing side by side with barren, burnt up fields of equal fertility in which no pains were taken to prevent evaporation. Ruskin has chosen Ethics of the Dust as the title of one of his most instructive writings.
See Desert; Sahara; Dune; Loess; Sand; Meteor; Volcano
It might be supposed there would be some place--for instance, amid the fresh white snows of St. Bernard Pass in Switzerland--where there would be absolutely no dust. But the men of science tell us otherwise. In fact the celebrated M. Jung collected snow at this very spot, 8,100 feet above the level of the sea; and when he had evaporated some fifteen liters of water produced by melting the snow, he found a very considerable quantity of dust; and this dust proved to be minute particles of iron!
Another scientist, M. Nordenskjold, in search of some place where dust would not be found contaminating the air, examined the greatest fall of snow in the memory of man (1871) at Stockholm, and likewise found a dust which proved to be metallic iron. Fearing that this might have come from neighboring roofs or chimneys, he had his brother examine the snow in a desolate plain surrounded by the forests of Finland. The black powder proved to be there also, and to be the same.
The fact is, the earth's atmosphere everywhere teems with dust. In the cities there is more than in the country; but not so much more as might be supposed. It has been estimated that the weight of dust suspended in the air above an ordinary city block is in the neighborhood of 33 pounds. At this rate the dust overhanging such cities as New York, Pittsburg, or Chicago would weigh several tons.
Ordinary city dust is made up of about three quarters cinders. The other quarter is organic matter. This means vegetable and animal, living and dead. For--unpleasant as is the thought--the dust contains its share of germs. Rains and snows reduce the quantity of dust in the air. In Paris it was found that an eight days' dry spell increased the quantity of dust in the air to three or four times the normal amount, which is from 6 to 8 milligrams to a cubic meter of air. Supposedly pure rain water will pick up dust from the air, so that a liter will yield all the way from 23 to 421 milligrams of dust.
Dust will travel marvelous distances in the air. On the 7th of February, 1863, there was a rain of sand in the Canary Islands, this sand coming from the Sahara desert 200 miles away. A more striking case was presented more recently when cinders from the great Chicago fire of 1871 arrived at the Azores forty days after that catastrophe. In the last century Europe experienced what was known as the celebrated "dry fog," which lasted for three months, and was found to have been caused by a volcanic eruption in Iceland.--Gibson Gardiner in The Technical World.