Page:Popular Science Monthly Volume 37.djvu/193

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ATMOSPHERIC DUST.

raises the dust is strong, nothing will be visible at the distance of a few yards, and the sun will be obscured. The dust penetrates everywhere, and can not be excluded from houses, boxes, and even watches, however carefully guarded. The individual particles of sand appear to be in such an electrical condition that they are ever ready to repel each other, and are consequently disturbed and carried up into the air. Dust columns are regarded by Dr. Cook as due to electrical causes. On calm, quiet days, when hardly a breath of air is stirring, and the sun pours down its heated rays with full force, little eddies arise in the atmosphere near the surface of the ground. These increase in force and diameter, catching up and whirling round bits of sticks, grass, dust, and lastly sand, until a column is formed of great height and considerable diameter, which usually, after remaining stationary for some time, sweeps away across country at great speed. Ultimately it loses gradually the velocity of its circular movement and disappears. In the valley of Mingochar, which is only a few miles in width, and surrounded by high hills, Dr. Cook, on a day when not a breath of air was stirring, counted upward of twenty of these columns. They seldom changed their places, and, when they did so, moved but slowly across the level tract. They never interfered with one another, and appeared to have independent existences. Mr. P. L. H. Baddeley, in his book on Whirlwinds and Dust Storms of India, tells of a gentleman at Lahore who fixed an electrometer apparatus, so adjusted as to report atmospheric electrical movements, and observed that it was strongly affected during dust storms.

Volcanic dust consists mainly of powdered vitrified substances reduced by the action of intense heat. It is interesting in many respects. The ashes or scoria shot out in volcanic eruptions are mostly pounded pumice, but they also originate from stones and fragments which are pulverized by striking against each other. Volcanic dust has a whitish-gray color, and is sometimes nearly white. Thus it is that, in summer, the terminal cone of the Peak of Teneriff e appears from a distance as if covered with snow; but there is no snow on the mountain at that season of the year, and the white cap of the peak is due to pumice ejected centuries ago.

The friction caused by volcanic stones and rocks as they are crushed in their collision develops a mass of electricity which shows itself in brilliant displays of branch lightning darting from the edges of the dense ascending column. During the great eruption of Vesuvius in 1822 they were constantly visible, and added much to the grandeur of the spectacle. It not unfrequently happens that the dust emitted from Vesuvius falls into the streets of Naples; but this is nothing in comparison with the mass of finely powdered material which covered and buried the towns of Pompeii,