Licence: Public Domain Mark
Credit: The atmosphere in relation to human life and health. Source: Wellcome Collection.
Provider: This material has been provided by the Augustus C. Long Health Sciences Library at Columbia University and Columbia University Libraries/Information Services, through the Medical Heritage Library. The original may be consulted at the the Augustus C. Long Health Sciences Library at Columbia University and Columbia University.
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![porous superficial l:i3'er, tuid csirries with it spores or minute organisms which have multiplied or germinated in the passages through which the vapor passes. A moderate degree of moisture aud rather free aera- tion of tlie soil are favorable to the growth of many kinds of micnjbes. We know that cotton wool, unless tightly jjackcd, will not stoj) the passage of microorganisms; sand and porous soil allow both air and water to ])ass without depositing all their ])articulat(! conlcnts. The filter beds of water comi)anies are ellicient not by the action of the sand, but by the retention of particulate bodies in the slimy covering Boon deposited above the sand. Cold nights following hot days seem to favor very much the exhala- tion of vapor from the earth. Wind may very likely have an effect in drawing out the gases from the soil, but this action is less imiwrtant to human health, for malarious germs are disi)ersed aud much less dangerous in windy \veather. Aitken has shown by his experiments on the formation of small, clear spaces in dusty air that bodies warmer than the air drive away dust from their surfaces and create the dust-free black envelope which sur- rounds them. He further showed that an evaporating surface has a .similar influence, and that dust was driven more than twice as far from the wet part of an object as from the dry, the object being above the temperature of the air. The necessary conditions for the repulsive eflect to be strongly shown are that the air must be acquiring heat and moisture from the surface. Yery little heat with moisture gives a thicker dark plane than double the heat would do. Dust passes through small openings with surprising ease; any opening which admits air allows the passage of the finest particles. The air contains enormous multitudes of pai'ticles so small that the concentrated light of the sun does not reveal them.^ We may fairly infer from these facts that no inconsiderable j^ait of the fine dust of the air, mineral and organic, is derived from below the surface of the ground. Some inter- esting experiments made a few years ago showed that the dust depos- ited in tightly closed cupboards is brought in by the movements of air induced by changes of temperature. Similarly, changes of tempera- ture must draw in and expel fine organic dust from and to air and soil. The present writer's observations led him to conclude that a great quantity of vapor issues from the earth even in dry weather, aud when the surface down to 2 inches or more is dry and dusty that the emission is very large in the evening, but that the maximum appears to take place in the early hours of the morning in dry weather; that soon after sunset in England in summer the temperature of short grass and con- tiguous air may be 9'^ to 15° or 20'=' colder than that of the earth at a depth of 1 to 15 inches, and that about sunrise the temperature of the top grass of a pasture field may be 20^ to 30^ colder than that of the earth at a depth of 9 to 15 inches and lower, and that the emission of 'Formatiou of clear spaces iu dusty air. By John Aitken, Proc. Roy. Soc, 1877. 230a 7](https://iiif.wellcomecollection.org/image/b21208724_0109.jp2/full/800%2C/0/default.jpg)
