COSMIC OR DUST. The fact that meteors entering the earth's atmosphere from interplanetary space are burned up and disappear leads to the rational conclusion that the result ing debris must remain in the atmosphere in the shape of very tine dust. or larger particles gradu ally descending to the earth's surface. In many cases the cloud left behind a meteor has remained visible for a relatively long period—in some cases three or four hours—during which it changes shape and diffuses, probably being il lumined by the sunlight of a distant twilight like the so-called phosphorescent clouds. In a few cases the dust collected by Nordenskjold from the surface of snow has been found to contain metallic iron and nickel. In still fewer eases hailstones have had a metallic nuclei's. Arago first suggested that these metallic dusts are the remnants of the shooting stars or meteors; they have therefore been spoken of as cosmic or meteoric dust. but in all cases except those of me tallic iron or nickel the ordinary mineral dusts undoubtedly conie from the earth itself. volcanic eruptions send immense volumes of dust into the air: in 1883 Krakatoa sent up four cubic miles of solid rock ground into impalpable dust. the finest ',articles of which floated about for several years. Besides the larger shooting stars and aerolites, there may well exist even the dust in the interplanetary space. some of which must frequently enter the earth's atmos phere: but except in the case of the niekel-iron compounds it seems at present impossible dis tinguish such cosmic dust from that which orig inates on the earth itself.
Pt sr WHIRLS AND Pr • S•-STORMS. The eddies of wind, especially those on a hot summer after noon. that form small whirls of ascending lint air. carry up leaves and dust to a considerable height. 'Under most favorable cireumstanees these may lie elevated to the level of the elonds, but under ordinary eircumstan•es they are ehar acteristie of the driest weather. and scarcely rise Ono feet before t hey subside. On the hot plains
of India and the western arid regions of the United States, which are subject to very high temperatures in the sunshine, the dust-column becomes a very tall column, with a rapid whirl ing motion, having a thin, hazy axis, due to the condensation of aqueous vapor in the region of low pressure that must mark the cent•al axis. Ube dust-storms of India have been pictured and described hest by Itaddelev in his hook under that name. published in London in 1880. Ile figures not merely isolated dust - whirls, hut groups of many such dancing around a central region. whence they 11 re known as 'dancing devils•' or remarkable combinations of whirls into figures illustrating most complex forms of vor tex motion, solute of which are stable and some unstable. These are all caused by the rapid ascent and whirl of air that is heated intensely by contact. with the hot soil and hot dust.
Dusr CouNrEtt. An iustrnuu nt invented by Mr. John Aitken for counting the total number of particles of dust in a small volume of air. In this instrument a definite quantity of air is sud denly and forcibly expanded. whereby iris cooled below the dew-point, and immediately a fog or haze is formed within it whose density depends upon the quantity of moisture and the quantity of dust that is present. Within a short time these partieles of fog descend to the bottom, falling upon a plate of black glass ruled off into small squares. With the help of a hand lens it is easy to count the number of particles per square. whence we calculate the number per cubic centimeter of air. Instead of this dust-counter. one may use Aitken's koniseope. in which we estimate the apparent cloudiness due to the dust. These instruments are described in his report in Bulletin No. 11 of the United States Weather Bureau.
The investigation of dust is an important branch of work in meteorology and in bacteriol ogy, and the whole subject is known as koniology.