ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH. 297 



porous superficial layer, and carries with it spores or minute organisms 

 which have multiplied or germinated in the passages through which 

 the vapor passes. A moderate degree of moisture and rather free aera- 

 tion of the soil are favorable to the growth of many kinds of microbes. 

 We know that cotton wool, unless tightly packed, will not stop the 

 passage of microorganisms; sand and porous soil allow both air and 

 water to pass without depositing all their particulate contents. The 

 filter beds of water companies are efficient not by the action of the 

 sand, but by the retention of particulate bodies in the slimy covering 

 soon 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 important to human health, for malarious 

 germs are dispersed and much less dangerous in windy weather. 



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 

 effect to be strongly shown are that the air must be acquiring heat and 

 moisture from the surface. Very 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. 7 ' The air contains 

 enormous multitudes of particles so small that the concentrated light 

 of the sun does not reveal them. 1 We may fairly infer from these facts 

 that no inconsiderable part 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, and 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 



'Formation of clear spaces in dusty air. By John Aitken, Proc. Roy. Soc, 1877. 



