STATE AGRICULTURAL SOCIETY. 113 



The mean annual temperature of the soil is the same as, or in 

 higher latitudes, a degree above that of the air. The nature and 

 position of the soil must considerably influence its temperature. 



The sources of that heat which is found in the soil are two, viz : 

 first, an internal one, the chemical process of oxydation or decay; 

 second, an external one, the rays of the sun. 



The heat evolved by the decay of organic matters is not incon- 

 siderable in porous soils containing much vegetable remains; but 

 this decay cannot proceed rapidly until the external temperature 

 has reached a point favorable to vegetation, and therefore this 

 source of heat probably has no appreciable effect one way or tlie 

 otlier on the welfare of the plant. The warmth of the soil, so 

 far as it favors vegetable growth, appears then to depend exclu- 

 sively on the heat of the sun. The circumstances which favor or 

 hinder the transmission or accumulation of the sun's heat, are 

 accordingly worthy of minute consideration. 



METHODS BY WHICH HEAT IS COMMUNICATED. 



1. Radiation of heat. — When we approach a liot body we per- 

 ceive its high temperature without touching it; heat streams from 

 it in all directions. This heat passes into the air and other 

 surrounding bodies; their temperature rises and that of the heat- 

 ed body falls; there is thus manifested a tendency to equaliza- 

 tion of temperature, and such a state is finally reached, after 

 which no more change of temperature is observed except some 

 hotter or colder body be introduced. In the day the sun radiates 

 heat towards the earth, and the latter becomes warmer; at night 

 the earth radiates heat into the planetary spaces, and itself grows 

 colder. All bodies are capable of radiating heat, but they possess 

 this property in very different degrees. 



The experimental results on this subject lead to no very defi- 

 nite general conchisions. It seems however, that the porosity, or 

 state of division of the surface of a body, has tlie princi])al 

 influence on its radiating power. The less dense the surface, the 

 greater its radiating power. Radiation seems to take place not 

 merely fruni the surface, but also from a little distance beneath it. 



2. Jihsorption of heat. — In our treatises on natural })hilosoj)hy, 

 there is much apparent confusion on this subject. Absorptive 

 power is often stated to be connected with the color of a body. 



[Ag. Trans.] H 



