682 THE SOUTHERN PLANTER. 



32°, so long as a fragment of ice is present. 

 The moment the ice disappears, the tem- 

 perature begins to rise again as before, at 

 the rate of one degree per minute. The 

 time during which the temperature of the 

 ice and water remains at 32°, is 140 min- 

 utes. During each of these minutes one 

 degree of heat enters the mixture, but is 

 not ■ indicated by the thermometer--the 

 mercury remains stationary ; 140° of heat 

 have thus passed into Ihe ice and become 

 hidden, latent, at the same time the solid 

 ice has become liquid water. The differ- 

 ence then between ice and water consists 

 in the heat that is latent in the latter. If 

 we now proceed with the above experi- 

 ment, allowing the heat to increase with 

 the same rapidity, we find that the tempe- 

 rature of the water rises constantly for 

 180 minutes. The therm(imeter then in- 

 dicates a temperature of 212° (32>< ISO,) 

 and the water boils. Proceeding with the 

 experiment, the water evaporates away, 

 but the thermometer continues stationar}^ 

 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 bring- 

 ing the steam into a cold space, water is 

 reproduced. If, by means of pressure or 

 cold, steam be condensed, the heat origi- 

 nally latent in it becomes sensible, /ree, 

 and capable of affecting the thermometer. 

 If, also, water be converted into ice, as 

 much heat is evolved and made sensible 

 as was absorbed and made latent. It is 

 seen thus that the processes of liquefac- 

 tion and vaporization are cooling process- 

 es ; for the heat rendered latent by them 

 must be derived from surrounding objects, 

 and thus these become cooled. On the 

 contrary, solidification, freezing, and va- 

 ])or-condensation are warming processes, 

 since in them large quantities of heat cease 

 to be latent and are made sensible, thus 

 w^armino; surroundino: bodies. 



From these facts we are able to under- 

 stand certain natural phenomena, whose 

 influence on veo^etation has been recoojniz- 

 ed from the earliest times. 



How does ike earth maintain its tempera- 

 ture — What are its relations to the sun's 

 heat— What is dew? — These are questions 

 we now come to consider. 



The earth has within itself a source of 

 heat, which maintains its interior at a high 

 temperature; but which escapes so rapid- 

 ly from the surface, that the soil would be 

 constantly frozen but for the external sup- 

 ply of heat from the sun. 



The direct rays of the sun are the im- 

 mediate cause of the warmth of the earth's 

 surface. When the sun shines most di- 

 rectly upon the earth, it is warmest, as at 

 summer mid-day. In a winter midnight 

 we have the greatest cold. The tempera- 

 ture of the soil near the surface changes 

 progressively with the season; but at a 

 certain depth the loss from the interior 

 and the gain from the sun compensate each 

 other, and as has been previously men- 

 tioned, the temperature remains unchang- 

 ed throughout the year. 



During a summer day the heat of the 

 sun reaches the earth directly, and it is 

 absorbed by the soil and the solid objects 

 on its surface, and also b}^ the air and wa- 

 ter. But these different bodies, and also 

 the different kinds of soil, have very dif- 

 ferent ability to absorb, or become warm" 

 ed by the sun's heat. It has before been 

 mentioned that air and water are almost 

 incapable of being warmed by heat applied 

 above them. Through the air especially, 

 heat radiates without being scarcely ab- 

 sorbed. The soil and solid bodies become 

 warmed according to their individual ca- 

 pacity, and from the air receives the heat 

 which w^arms it. From the moist surface 

 of the soil goes on a rapid evaporation, 

 which renders latent a large amount of 

 heat, so that the temperature of the soil is 

 not rapidly but gradually elevated. The 

 ascent of water from the sub-soil to supply 

 the place of that evaporated, goes on as 

 before described. The liquid water of the 

 soil has combined with (rendered latent) 

 avast amount of heat therefrom, and pass- 

 ed as gaseous water (vapor) into the air. 

 When the sun declines the process dimin- 

 ishes in intensity, and when it sets, the 

 reverse takes place. The heat that had 

 accumulated on the surface of the earth 

 radiates into the cooler atmosphere and 

 planetary spaces, the temperature of the 

 surface rapidly diminishes, and the air it- 

 self becomes cooler by convection. As 

 the cooling goes on, the vapor suspended 

 in the atmosphere begins to condense upon 

 cool objects, while its latent heat becom- 

 ing free hinders the too sudden reduction 



