DUST (AS. dust, Ger. Dust, connected with Ger. Dunst, vapor, 01 dun ist, tunist, breath, storm. Cloth, da uns, odor. Skt. (Divans. 01 r(IS, to fall to dust. to perish). A•mosmi•uc. Tt is a familiar fact that dust is always present in the atmosphere. nut merely in houses and near the ground out of doors, but over the (wean and even on the tops of mountains. hl order to ob tain air absolutely free from dust, it may be filtered slowly through plugs of col tom or it may be suddenly expanded so as to cool below the dew-point, when a portion of its moisture will condense upon the particles of dust and ea use them to settle quickly to the bottom of the containing vessel. When a beam of sunlight shines through dustless air, nothing is seen by an observer standing at one side, but if dust is present tlme sunlight is scattered in all di reel ious mud the b'aiir of light is said to become visible. \\ lien atmospheric dust is collected and examined under a nnic•ruseope, it is seen to emmsi.t largely of grains of pollen. bits of vegetable libre, and hairs :rnd all kinds of mineral frarurent_., iuelmd ing uerasiou:mlly metallic iron. \\ hen, on the other hand. dust is allowed to settle upon prop erly prepared vegetables, or the jellies and broths used fur 'cultures' in baeteriulogv, and is kept in a warm room• there usually springs up is a few days a vigorous growth of fund and bacteria, showing that living germs were present. although they nmay havr brtm iwisih1e in the ordinary nmicroseope. Such dust gives rise in nature to growths that were formerly spoken of as spon taneous ma-nerat ion : but it is now abundantly demonstrated that they .imply prove the precious existence of living germs. Some of these germs are highly iu,jurious to men and animals. and are the sources of epidemic diseases; others. when brought to tlme ,round by means of rain, may germ in ito on tlme wet soil and produec ch:n•ae teristir fungi. These are sometimes black, giving rise to what is cal led black rain (q.v.) ; sometimes red, giving rise to red rain or blood rain and occasionally green snow or red snow. \\hen yel low pollen is brought dawn and spread on the wet ground, it is often spoken of as a rain of sulphur. No fernten tat iin, souring, festering, or other trouble e:nsed by the development of bac teria and fungi eau originate iu absolutely pure air.
Showers of dust occur in connection with ac tive volcanoes, prairie fires, forest fires, torna does, and strong winds, that elevate the dust in one region only to deposit it in another. llrdi nary storm-winds sweepin; over a forest and rob lung the trees of their pollen produee showers of sulphur ur other forms of dust within a few hmumdreml nines. Pem:rkablc dust-showers orig
inate in the region of the Sahara in North Afriea, This dust, which is execediugly fine anml usually red, has been transported 1200 miles to the north across Ilse JlediterraocalW into Europe. (Fee Ilene description of the (lust-storm of February 25, 187), on page 34 of Rmnrtlmo1ummo mr's .Itlas of Mete oroluql/, and that ut the red dust of 1901 in Nn turr). .\huost as far to the west and southwest of the Sahara a tine white dust falls on vessel, in the Atlantic 1lceau dining the so-called harmat tan fy.cJ- or very dry winds blowing out' from the interior of the desert region, Exanmivation of this dint shows it to he almost exclusively composed of the tests of diotoms—rmcroscopim organisums, growing on the veg'tation in the fresh of the interior. 'I'lme formation known in geology as loess (q.v.l is an exeeedingh• fine. fri able soil, easily raised try the wind and carried to great distances. This format ion exists in China. in layers of several hundred feet in depth, and in the prairie region of North Anueriea to a depth of a hundred feet or less, :rod is supposed by many to owe its origin entirely to tlie dust eaurricd by the wind in certain' seasons of the year. A shower of black snow due to this dust is described in time La ml ed Moles lIunt/ml1/ II'cotlu-r Rem ten' for .lams• ary, Iti93. pp. 13-19. Armmorrg the most remark able shower: of dust and a.slses are those that have attended volcanic eruptions, such as Those 01 \"esuviu°. The cloud of dust. proceeding from Skaptar .T7iku11 in Iceland during the eruption of Slay 'LIT to ,Tune 15, 1573, extended southeast ward 2000 miles, over all of Europe and still 1mnt lie r into Syria. The dust from an eruption of ('otopaxi was calculated by Sir. \\ lnvunper to represent 2,1)00.000 tons of material; that from the volcano Soufrii i-e in the island of Saint \\est lrmmlios, on April 30, 1812, fell on Barbados, 50 miles to the east• to the depth of one inch, and ostended thence eastward be vund the horizon during Slav 1st. The great eruption of ashes and Vapor from Krakatoa in the Straits of Sunda, during August 26 and 27 spread a cloud of dust and vapor over tlue whole globe between latitudes 10° S. and 6l1° N., some of which, renm:mining at a great height. was observable three year•.. later. The dust collected 900 moil's west of the volcano was similar in c'oornpasiIimmn to that collected only 1(l0 miles distant. _\ great fall of dust attended tire eruptions on Slam rlinique and Saint. Vincent in 1902, and produced .striking sunset glows, .similar to those that were developed by the ejected ashes flom Krakatoa.