lft>! HOVr CROPS FEEP. 



■water are almost incapable of being warmed by heat ap- 

 plied above them. Through the air, heat radiates without 

 being absorbed. Solid bodies whicli 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 plnce of that evaj^orat- 

 ed, goes on as before described. When the sun declines, 

 the process diminishes in intensity, and w^hen it sets, the 

 reverse takes place. The heat that had accumulated on 



* When a piece of ice is placed in a vessel whose temperature is increasing, 

 by means of a lamp, at tlie rate of one desree of the thermometer every minute, 

 it will be found that the temperature of the ice rises until it attains 32°. When 

 this point is reached, it begins to melt, but doos not suddenly become fluid : the 

 melting goes on very gradually. A thermometer placed in the water remains 

 constantly at 32° 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 degree per 

 minute. The time during which the temperature of the ice and water remains 

 at 33° 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 dift'erence, then, 

 between ice and water consists in the heat that is latent in the latter. If we now 

 proceed with the above experiment, allowing the heat to increase with the same 

 rapidity, we find that the temperature of the water rises constantly for ISO min- 

 utes. The thermometer then indicates a temperature of 212°, (32+180,) and the 

 water boils. Proceeding with the experiment, the water evaporartes 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 ol 

 pressure or cold, steam be condensed, the heat originally latent in it becomes 

 sensible,/?-«A 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 tvarminff processes, since in them 

 large quantities of heat ce-ise to be latent and are made sensible, thus warming 

 purroundiug bodies. 



