DUST. 548 jjresent the sunlight is scattered in all directions iiud the beam of light is said to become visible. When atmospheric dust is collected and examined under a microscope, it is seen to consist largely of grains of pollen, bits of vegetable libre, and hairs and all kinds of mineral fragments, includ- ing occasionally metallic iron. When, on the other hand, dust is allowed to settle ujion jirop- erly prepared vegetables, or the jellies and broths used for "cultures' in bacteriology, and is kept in a warm room, there usually springs up in a few days a vigorous growth of fungi and bacteria, showing that living germs were present, although they may have been invisible in the ordinary microscope. Such dust gives rise in nature to growths that were formerly spoken of as spon- taneous generation ; but it is now abundantly demonstrated that they simply prove tlie previous existence of living germs. Some of these germs are highly injurious to men and animals, and are the sources of epidemic diseases; others, when brought to the ground by means of rain, may germinate on the wet soil and produce charac- teristic fungi. These are sometimes black, giving rise to what is called black rain (q.v. ) : scmietimes red, giving rise to red rain or blood rain and occasionally green snow or red snow. .When yel- low pollen is brought down and spread on the wet ground, it is often spoken of as a rain of sulphur. No fermentation, souring, festering, or other trouble caused by the development of bac- teria and fungi can originate in 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. Ordi- naiy storm-winds sweeping over a forest and rob- bing the trees of their pollen produce showers of sulphur or other forins of dust within a few hundred miles. Remarkable dust-showers orig- inate in the region of the Sahara in North Africa. This dust, which is exceedingly fine and ustially red. has been transjjortcd 1200 miles to the north across the Jlcditerranean into Europe. ( Sec the description of the dust-storm of February 25, 1870. on page 34 of Bartholomeic's Atlas of ilete- orohyy, and that of the red dust of 1001 in Xa- ture). Almost as far to the west and soutlnvest of the Sahara a fine white dust falls on vessels in the Atlantic Ocean during the so-called harmat- tan (q.v.), or very dry winds blowing out- from the interior of the desert region. Examination of this dust shows it to be almost exclusively composed of the tests of diatoms — microscopic organisms, growing on the vegetation in the fresh ■water of the interior. The formation known in geology as loess (q.v.) is an exceedingly fine, fri- able soil, easily raised by the wind and carried to great distances. This fonnation exists in China in layers of several hundred feet in depth, and in the prairie region of North America to a depth of a hundred feet or less, and is supposed by many to owe its origin entirely to the dust carried by the wind in certain' seasons of the year. . shower of black snow due to this dust is described in the United States Monthly Wc<ilher Review for Janu- ary, 1895, pp. 15-19. Among the most remark- able showers of dust and ashes are those that have attended volcanic eruptions, such as those of Vesuvius. The cloud of dust proceeding from Skaptar .Tiikull in Iceland during the eruption of May 20 to June 18, 1873, extended southeast- DUST. ward 2000 miles, over all of Europe and still further into Syria. The dust from an eruption of Cotopaxi was calculated by Mr. hymper to represent :!.000,000 tons of material ; that from the volcano SoufriJre in the island of Saint 'incent. West Indies, on April 30, 1812, fell on Barbados, 50 miles to the east, to the depth of one inch, and extended thence eastward be- yond the horizon during May 1st. The great eruption of ashes and vapor from Krakaloa in the Straits of Sunda, during August 2G and 27, 1883, spread a cloud of dust and vapor over the whole globe between latitudes 10° S. and 60° N., some of which, remaining at a great height, was observable three years later. The dust collected 900 miles west of the volcano was similar in composition to that collected only 100 miles distant. A great fall of dust attended the eruptions on Martinique and Saint Vincent in 1902, and i)roduced striking sunset glows, similar to those that were developed by the ejected ashes from Krakatoa. The great importance of dust in the formation of cloud and rain has been especially elucidated by the labors of Mr. .John Aitken, of Falkirk, Scotland, who has shown that in the ordinary process of the formation of cloud or fog by cool- ing air the condensation of the moisture takes place first upon the particles of dust as nuclei; that, in fact, every particle of dust collects moi_sture, and that every particle of cloud or fog implies a nucleus of dust. On this fundamental principle Aitken has based the construction of his so-called dust-counter, which he has made quite portable, and which enables one to deter- mine with close proximity the relative and the absolute dustiness of any sample of air. He has made such determinations in many parts of Eu- rope and in a great variety of places. The dustiest air is found in the cities; thus, in Lon- don. Edinburgh, and Paris there are from 80,000 to 210,000 particles per cubic centimeter (about the fifteenth part of a cubic inch). The air hav- ing least dust, so far as yet obser-ed, is that of the western Highlands of Scotland — namely, from 7(100 down to IG particles per cubic centimeter; the air of the Swiss mountain-tops has a similar small content of dust. Aitkcn"s investigations have explained quantitatively that which was be- fore known only qualitatively — namely, that the air most soothing to those who are troubled with delicate lungs is that which has the least dust. According to these results, the presence of dust makes it easier for cooling air to form cloud or fog. Cloudy or foggy air must be con- sidered as dustless air in which the dust previous- ly existing is now replaced by minute fioating globules of water. If, now, this dustless but foggy air is still further cooled, its surplus aqueous vapor, which does not easily condense upon the globules already present, supersaturates the dust- less air, thus bringing about a condition of un- stable equilibrium. This condition has been espe- cially investigated by IMr. C. T. R. Wilson, of the Cavendish Laboratory. Cambridge, England, who has shown that condensation eventuallv takes place, but only after a considerable addi- tional cooling and with violence. In this process latent heat is set free and phenomena produced whirh imitate those observed in the interior of thunder-clouds from which rain is falling. It would seem plausible to conclude, therefore, that whereas dustv air is necessary for the fonnation
