OJi the Temperature of Soils. 
123 
colder at the time of the commencement of vegetation, and 
throughout the summer, than well drained or naturally drier lands. 
If we knew the capacity for heat of any given soil, and the weight 
of water mixed with it in excess, over the proper complement ne- 
cessary for vegetation, it would be easy to determine, very nearly, 
the depression of temperature caused by its evaporation. We 
know that the heat of a pound of water in its gaseous state, that 
is, as steam, would raise the temperature of about 1000 lbs. of 
water one degree ; so that if the specific heats of the solid and 
fluid bodies were alike, the evaporation of a pound of water would 
keep down the temperature of 1000 lbs. of earth one degree; of 
500 lbs., two degrees ; and so on. 
Secondly ; excess of humidity obstructs the absorption of heat 
by the solid matter of the soil. Water in a quiescent state is one 
of the worst conductors of heat with which we are acquainted. 
If it be warmed on the surface — and it derives, when mixed with 
soil, nearlv all its heat from the sun's rays — water transmits little 
or no heat downwards. 
If a mass of water be heated from below, the whole quickly 
attains an uniform temperature by reason of the motion excited 
amongst its particles. The lowest stratum, when heated, becomes 
of less specific gravity than that resting upon it, and the header 
superincumbent portions descend and push that which has been 
warmed upwards. In this manner rapid circulation is induced. 
If, on the contrary, it be heated from above, i. e. on the surface, 
the film of warmed water floats on the top, by \'irtue of its superior 
levity, and no heat is conveyed below : there is no circulation 
from above downwards. Much of the heat of the sun's rays is, 
therefore, prevented by excess of water from entering into, and 
being transmitted through, the mass of the soil. 
Thirdly ; water is a powerful radiator of heat, t. e. it cools 
quickly. All bodies, whether fluid or solid, possess peculiar 
powers of emitting or radiating heat, and water was esteemed by 
the late Professor Leslie — in which opinion he has been joined by 
other philosophers — to stand at the head of radiating substances. 
The phenomena of the production of cold by radiation and 
evaporation are elegantly exemplified by the well-known experi- 
ment of exposing water, warm enough to give off visible vapour, in 
one saucer, and an equal bulk of water drawn from a well in 
another saucer. The former, on a sharp frosty morning, will be 
found to exhibit ice the soonest.* The cooling powers of evapo- 
ration and radiation combined, and of radiation chiefly, or solely, 
are represented in this experiment by the order of congelation in 
• Boiling water thrown on the ground will freeze sooner than cold 
water. 
