STATE AGRICULTURAL SOCIETY. 117 



tence 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, /ree, and capable of afifecting the thermome- 

 ter. 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 liquefaction and vaporization are cooling 

 processes; for the heat rendered latent by tliem must be derived 

 from surrounding objects, and thus these become cooled. On the 

 contrary, solidification, freezing, and vapor-condensation are 

 warming processes, since in them large quantities of heat cease to 

 be latent and are made sensible, thus warming surrounding bodies. 



From these facts we are able to understand certain natural 

 phenomena, whose influence on vegetation has been recognized 

 from the earliest times. 



How does the earth maintain its temperature — What are its rela 

 lions to the sun's heat — What is dewl — 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 rapidly 

 from the surface, that the soil would be constantly frozen but for 

 the external supply of heat from the sun. 



The direct rays of the sun are the immediate cause of the 

 warmth of the earth's surface. When the sun shines most directly 

 upon the earth, it is warmest, as at summer mid-day. In a win- 

 ter midnight we have the greatest cold. The temperature of the 

 soil near the surface changes progressively with the season; but 

 at a certain de2:)th the loss from the interior and the gain 

 from the sun compensate each other, and as has been previ- 

 ously mentioned, the temperature remains unchanged 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 

 surlace, and also by the air and water. I3ut these different bodies, 

 and also the different kinds of soil, have very different ability to 

 absorb, or become warmed 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 absorbed. The soil and 



