296 ATMOSPHERE IN RELATION TO HUMAN LIFE AND HEALTH. 



from vapor derived from the earth. A large quantity of dew was inva- 

 riably found on clear nights in the interior of closed vessels inverted 

 over grass and sand, very little or none in vessels inverted over plates 

 lying on the ground. The inverted glasses or vessels, however much 

 their rims were embedded in the ground, gave similar results. More 

 dew was found on the lower surface of plates of glass or earthenware 

 or boards slightly raised above the ground than on the upper surface. 

 The lower sides of stones, slates, glass, and paper on the ground were 

 more bedewed than the upper sides. The lower half of stones lying 

 or embedded in sand was more often bedewed and frosted than the 

 upper half. The interior of closed vessels inverted on the grass and 

 covered with two other vessels of badly conducting substance was 

 thickly bedewed, and the grass in the three inclosures was also thickly 

 bedewed. The deposit on the interior of vessels was much less over 

 dry garden earth than over sand or turf. A great deal of dew was 

 deposited on the interior of vessels over dry sand or dust, the earth 

 being somewhat moist an inch or two inches below. Pebbles, etc., 

 lying on a dusty road became quite wet underneath early in the evening, 

 and over grass the underside of a square of glass is clouded soon after 

 the grass loses the sunshine. A very great difference of temperature 

 was found soon after sunset after hot days between the temperature of 

 the soil at a depth of 2 or 3 inches and the temperature of the air close 

 to the ground, just above the blades of grass. On one evening at 

 11 p. m. the temperature of the exposed grass was 36; of the soil at 15 

 inches, 60.5. 



The author became convinced by these experiments and other con- 

 siderations, that a great deal of dew comes from vapor from the soil 

 and from plants, and at sea from vapor from the surface of the sea; that 

 malaria and some other diseases are largely caused by emanations from 

 the soil at night bearing organisms into the air, which are then retained 

 by the damp air in a cold stratum near the ground, and that sand over- 

 lying damp earth permits air and vapor to rise easily through it. Also, 

 it became evident that a great deal of soil-air may be drawn into houses 

 through pervious soil, and that the neighborhood of damp ground may 

 be thickly infected with organisms contained in the air and vapor which 

 emerge from the soil. A dry covering of sandy earth is not only little 

 impediment to the exhalation of vapor, but may serve to protect micro- 

 organisms from the killing action of dry air and sunshine. 1 



EXHALATION OF GASES AND PARTICLES FROM THE EARTH. 



It is generally assumed that evaporation or distillation of water gives 

 rise to pure vapor and leaves behind all impurities, but, as a matter of 

 fact, in many natural conditions this is far from being the case. When 

 earth becomes heated, moisture forces its way as a vapor through a 



'The author has treated this subject more fully in Trans. Sanit. Ins. for 1892: The 

 Exhalation of Vapor from the Earth. 



