DUST 



1887 



DUST 



takes place very gradually. The dust particles 

 cool more rapidly than the gaseous parts of 

 the air, and each particle becomes the nucleus 

 of a droplet of mist, the masses of mist 

 form cloud, from the lower surface of which 

 the mist gathers into rain drops. Were it not 

 for this beneficent provision of nature almost 

 every rainstorm would be a fierce deluge of 

 down-pouring water out of a cloudless sky. 

 There would be no gentle summer showers; 

 and if there were clouds the clouds would be 

 of a form and composition now unknown. A 

 knowledge of the nature of the dust content 

 of the atmosphere is quite as important as 

 that of the water-vapor content, for, in a way, 

 it has quite as much to do with the distribu- 

 tion of rain. It is not far from the truth, 

 therefore, to say that atmospheric dust is the 

 parent of cloud, just as cloud is the parent of 

 rain. 



Two Kinds of Atmospheric Dust. Broadly 

 speaking, atmospheric dust may be classed as 

 consisting of two kinds: the dust motes or 

 floating matter of the air, and the flying dust 

 that is swept from bare surfaces of the ground 

 and carried into the air by the wind. A dis- 

 tinction of this sort, however, is not exact; 

 it is merely one of convenience, for there are 

 dust motes that settle to the ground, and par- 

 ticles of flying dust that practically do not. 

 Theoretically, all matter not in gaseous form is 

 supposed to settle to the ground; practically, 

 much of it remains in suspension for an indefi- 

 nite length of time. Every fall of rain or of 

 snow brings these particles to the ground in 

 numberless millions; electrification and winds 

 drive them into the air again. If the earth and 

 the dust motes are similarly electrified the lat- 

 ter are repelled. At such times they accumu- 

 late until the air next the ground becomes 

 more or less hazy. 



Mr. John Aitken, a Scotch meteorologist, has 

 devised a very ingenious method of estimating 

 the number of dust motes in a given volume 

 of air. A measured quantity of air confined 

 in a receiver is suddenly cooled below the dew 

 point. The moisture of the air condenses on 

 the dust motes and they fall upon a plate of 

 black glass ruled in such a manner that the 

 droplets may be readily counted with a strong 

 magnifying glass. Mr. Aitken finds that if the 

 air contains not more than 8,000 dust motes 

 per cubic inch, it is fairly clear and the out- 

 lines of distant objects are plainly visible. On 

 the other hand, when about four times that 

 number are present, the air is very hazy. On 



a very clear day following a shower, the num- 

 ber may be less than 4,000 per cubic inch; 

 ordinarily, however, the numbei is not far 

 from 8,000 to 12,000 per cubic inch. 



Sources of Dust Motes. Dust motes come 

 from various sources. Undoubtedly some are 

 particles of wind-blown earthy matter that by 

 long attrition, or wearing away, have become 

 so minute that they remain suspended in the 

 atmosphere until brought down during a 

 shower. A large proportion are of volcanic 

 origin. The eruption of Krakatoa, in Sunda 

 Strait, ejected so much floating matter into 

 the atmosphere that red sunsets and hazy con- 

 ditions caused by it lasted for several years. 

 The chimney stacks out of which the burnt 

 products of ten billion tons of coal are yearly 

 poured also account for a considerable pro- 

 portion of the motes. The wind-whipped sand 

 of the sea furnishes its quota, and the pro- 

 portion is not small. The "chlorine zones" 

 which so many boards of health in seacoast 

 cities are now studying are strips of seacoast 

 in which salt prevails in the air. But many 

 dust motes are of meteoric origin that is, they 

 are matter which, for want of a better name, 

 may be called planetary dust, which the earth 

 gathers from space. Some of these particles 

 are very much like the lava of terrestrial vol- 

 canoes; some are spherules of iron. The lat- 

 ter may be gathered by a magnet from the 

 dust accumulations of undisturbed niches and 

 corners; they may be gathered also upon a 

 microscope slide which rests upon a magnet. 

 In any case the difficulty in examining them 

 is considerable, and the slightest jar will cause 

 them to fly into the air. 



Dust motes apparently do not affect the 

 sanitary condition of the air. There is no 

 evidence to show that the health of a com- 

 munity is affected by them. Because they are 

 such important factors in the production of 

 rain, dust motes should be considered, not as 

 "foreign matter of the air," but as a real and 

 essential part of it ; moreover, without a knowl- 

 edge of the part they play, the study of 

 atmospheric moisture is incomplete. 



Flying, or Wind -Blown, Dust. The flying, 

 or wind-blown, dust of the atmosphere at first 

 thought might appear to differ from the dust 

 motes only in the size of the particles; as a 

 matter of fact, however, the work they per- 

 form in nature is vastly different. Dust motes 

 exist at a height of ten miles or more in the 

 air; flying dust rarely is carried higher than a 

 few hundred feet. Yet in that low altitude 



