ATMOSPHERIC BUST. 181 



raises the dust is strong, nothing will be visible at the distance of 

 a few yards, and the snn will be obscured. The dnst penetrates 

 everywhere, and can not be excluded from honses, 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 diam- 

 eter, catching up and whirling round bits of sticks, grass, dust, 

 and lastly sand, until a column is formed of great height and con- 

 siderable 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 dis- 

 appears. 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 inter- 

 fered with one another, and appeared to have independent exist- 

 ences. 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 elec- 

 trical movements, and observed that it was strongly affected dur- 

 ing 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 erup- 

 tion of Vesuvius in 1822 they were constantly visible, and added 

 much to the grandeur of the spectacle. It not unfrequently hap- 

 pens 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, 



