DUST, the fine particles of matter which float on currents of air or settle on surrounding objects. These particles consist of sand, soot, cotton fibre, pollen, fine hair, pulverized excreta of animals, parts of seeds, bacteria, molds, etc. This is the ordinary condition of things. Fre qu4ntly, as was the case after the Krakatoa eruption in 1883, the atmosphere becomes widely and densely charged with volcanic particles of varied minuteness, the smallest and lightest par ticles of which must take months, even years, in settling. The dust from this eruption circled round .the entire globe several times, and for two years it gave rise to extraordinary sunset glows and other remarkable optical phenomena. The finest particles are believed to have been raised to an altitude of 50 miles and to have remained buoyed up by air currents for several years. The presence of an abundance of pollen in the air accounts for the occurrence of °sulphur showers or °golden rain' The edry fog' of 1783, which covered Europe for three months, is regarded as having been due to an eruption in Iceland.
On 8, 9 and 10 March 1901 heavy dust storms arose in the desert region of southern Algeria, were carried north by a great cyclonic storm, leaving deposits as far north as Denmark, Rus sia and Finland, the last traces of the storm settling in those countries on the 12th of that month. The extreme distance carried was 2,500 miles ; the total amount falling on the Continent being estimated at 1,800,000 tons, two-thirds of which fell south of the Alps, and as Much more is supposed to have fallen in the Mediterranean. Off the west coast of Africa between the Canaries and Cape Verde Islands, haze dust due to storms from the Sahara Desert is frequently encountered by vessels. The dust storms in certain parts of India are occasioned by accumu lations of whirlwind columns moving en manse or in rapid succession over the earth's surface in a direct or wavy line. When passing over the arid plains these whirl high into the at mosphere great quantities of dust, which are rolled onward in huge volumes, forming at times a sublime spectacle.
Conflagrations are responsible for the spreading of dust over very extensive areas. Forest fires along Lake Champlain and down to the neighborhood of Ticonderoga caused a night-like darkness to fall on New England on 19 May 1780, the unusual phenomenon suggest ing to the inhabitants the coming of the end of the world. Cinders from the great fire in
Chicago spread over a very extended area, and reached the Azores 40 days after the outbreak of the conflagration. The burning of peatbogs in the barrens of Labrador is supposed to have been the cause of the "yellow day" in New England (6 Sept. 1881), when the atmosphere had an intensely brassy appearance. The dust of such meteorites as undergo complete combustion on contact with atmospheric air has been traced on the earth's surface; fine ferruginous par ticles are often found on the snowy summits of high mountains and on the icefields of Green land and other northern regions. This is called by scientists cosmic dust.
The dust with which the atmosphere is im pregnated, especially in cities, is not only in jurious to health, but causes loss in business, in addition to which certain kinds are liable to explosion. Damage from dust and soot in departmental stores is a definite loss item, caus ing depreciation in stock and high charges for dusting and cleaning. The drastic remedy lies in the washing of the air, warmed in winter and cooled in summer, with the humidity ad justed for comfort and health. In such build ings the windows will remain unopened — the open window being regarded as a source of contamination. Air was cleaned at first by passing it through a cheese cloth screen, but a common practice now is to subject it to water sprays, which remove as high as 98 per cent of dust and other impurities, after which it is circulated throughout the building. In the' running of pneumatic hammers, drillers and riveters, dynamos and other machines, the elim ination of dust where practicable is a great economy which materially lengthens the 'life of the machinery. In mines, flour mills and other places where dusty operations'are carried on, there is constant danger from dust' ex plosions. The amount of dust that must be present to cause an explosion naturally varies to some extent. Among dusts which propagate flame readily and ignite so easily that an ex plosion would certainly be produced by lighting a match in the dust-laden space are sugar, starch, rice refuse, fine wood dust, malt, oat husks and grain in flour mills. Dusts from leather, shoddy and shellac compositions and sawdust, while they are readily ignited, are not so dangerous, as they ordinarily require a source of heat of larger size and higher temperature or a source that acts for a considerable time.