188 HOW CROPS FEED. 
water are almost incapable of being warmed by heat ap- 
plied above them. Through the air, heat radiates without 
being absorbed. Solid bodies which have dull and porous 
surfaces absorb heat most rapidly and abundantly. The 
soil and solid bodies become warmed according to their 
individual capacity, and from them the air receives the 
heat which warms it, From the moist surface of the soil 
goes on a rapid evaporation of water, which consumes * a 
large amount of heat, so that the temperature of the soil 
is not rapidly but gradually elevated. The ascent of wa- 
ter from the subsoil to supply the place of that evaporat- 
ed, goes on as before described. When the sun declines, 
the process diminishes in intensity, and when it sets, the 
reverse takes place. The heat that had accumulated on 
* When apiece of ice is placed in a vessel whose temperature is increasing, 
by means of a lamp, at the rate of one degree of the thermometer every minute, 
it will be found that the temperature of the ice rises until it attains 82°. When 
this point is reached, it begins to melt, but does not suddenly become fluid: the 
melting goes on very gradually. A thermometer placed in the water remains 
constantly at 82° so long as a fragment of ice is present. The moment the ice 
disappears, the temperature begins to rise again, at the rate of one degreé¢ per 
minute. The time during which the temperature of the ice and water remains 
at 82° is 140 minutes. During each of these minutes one degree of heat enters 
the mixture, but is not indicated by the thermometer—the mercury remains sta- 
tionary; 140° of heat have thus passed into the ice and become hidden, latent; 
at the same time the solid ice has become liquid water. The difference, then, 
between ice and water consists in the heat that is latent in thelatter. If we now 
proceed with the above experiment, allowing the heat to incrcase with the same 
rapidity, we find that the temperature of the water rises constantly for 180 min- 
utes. The thermometer then indicates a temperature of 212°, (82+-180,) and the 
water boils. Proceeding with the experiment, the water evaporates away, but 
the thermometer continues stationary so long as any liquid remains. After the 
lapse of 972 minutes, it is completely evaporated. Water in becoming steam 
renders, therefore, still another portion, 972°, of heat latent. The heat latent in 
steam is indispensable to the existence of the latter. If this heat be removed 
by bringing the steam into a cold space, water is reproduced. If, by means of 
pressure or cold, steam be condensed, the heat originally latent in it becomes 
sensible, free, and capable of affecting the thermometer, If, also, water be con- 
verted into ice, as much heat is evolved and made sensible as was absorbed and 
made latent. It is seen thus that the processes of liquefaction and vaporization 
are cooling processes ; for the heat rendered latent by them must be derived from 
surrounding objects, and thus these become cooled. On the contrary, solidifica- 
tion, freezing, and vapor-condensation, are warming processes, since in them 
Jarge quantities of heat cewse to be latent and are made sensible, thus warming 
surrounding bodies. 
