on the Temj)erature of 'Soils. 
131 
Wells ascertained, among other phenomena affecting the tempe- 
rature of soils, that the attraction of substances for water is not 
exactly proportional to their radiating energy ; and that — 
".the formation of dew not only does not produce cold, but, like every 
precipitation of water from the atmosphere, produces heat." 
As the earth becomes colder than the atmosphere on dewy 
nights, by reason of its radiating energy, and as the moisture sus- 
pended in the latter possesses the atmospheric temperature, dew, 
with respect to the surface of the earth, is warm. Were it not that 
this antagonist warming process counteracts, on cloudless and serene 
nights, the rapid escape of heat from the earth by rachation, it is pro- 
bable that the temperature of the soil would be depressed, during 
the sun's absence, by a greater amount than it is elevated during its 
presence ; and that the extremes of heat and cold, or the %-icissitudes 
of temperature, during 24 hours, might be so great as to destroy 
vegetable life in the summer season. The least experienced ob- 
server may easily satisfy himself of the superior cold of the earth's 
surface on clear nights, relatively to that of the atmosphere. 
Hoar-frost, which is frozen dew, frequently forms on grass when 
the thermometer in the air indicates a temperature some degrees 
higher than the freezing point ; a phenomenon showing that the 
earth, or the leaves of plants, were colder than the atmosphere, 
and below the freezing point, when the deposition took place. In 
Bengal, ice is (or was) procured artificially, on a large scale, and 
for profit, by exposing water to the sky in porous earthen pans 
placed in shallow pits. The difference of temperature between 
the air and the water, at the time of its congelation, has often been 
observed, on clear serene nights, to amount to 14°, and even 16°. 
The air near the ground must then have had a temperature of 
about 46° or 48°. 
The genius of Davy would appear to have almost divined the 
mystery of dew-making, even before the complete revelation of 
its true and only cause by Dr. Wells, as may be gathered from 
the following profound remark : — 
"The power of soils to absorb water from air is much connected with 
fertility. When this power is great, the plant is supplied with moisture 
in dry seasons ; and the effect of evaporation in the day is counteracted 
by the absorption of aqueous vapour from the atmosphere, by the 
interior parts of the soil during the day, and by both the exterior and 
interior during the night." — Agricultural Chemistry. 
It a soil be sufficiently permeable to air, and not saturated with 
water, it is in a state to receive accessions of moisture from the 
atmosphere, which is a constant and inexhaustible vehicle of humi- 
dity ; and if the temperature of a sufficiently porous subsoil be at, 
or below the dew-point, as will frequently be the case during 
some portion of the day, in the summer season, the process of de- 
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