680 



THE SOUTHERN PLANTER. 



is higher in day time than that of the air ; 

 at night the temperature of the surtace rap- 

 idly falls, especially when the sky is clear. 



In temperate climates, at a depth of 

 three feet, the temperature remains un- 

 changed from day to night; at a depth of 

 20 feet the annual temperature varies but 

 a degree or two ; at 75 feet below the 

 surface, the thermometer remains perfectly 

 stationary. In the vaults of the Paris 

 Observatory, 80 feet deep, the temperature 

 is 50° Fahr. In tropical regions the point 

 of nearly unvarying temperature is reached 

 at a depth of one foot. 



The mean annual temperature of the 

 soil is the same as, or in higher latitudes, 

 a degree above that of the air. The na- 

 ture and position of the soil must conside- 

 rably 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 or- 

 ganic matters is not inconsiderable in po- 

 rous soils containing much vegetable re- 

 mains ; 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 the 

 other on the welfare of the plant. The 

 warmth of the soil, so far as it favors veg- 

 etable growth, appears then to depend ex- 

 clusively on the heat of the sun. The 

 circumstances which favor or hinder the 

 transmission or accumulation of the sun's 

 heat, are accordingly'' w^orthy of minute 

 consideration. 



METHODS BY WHICH HEAT IS COMMUNI- 

 CATED. 



1. Radiation of heat. — When we ap- 

 proach a hot body we perceive its high tem- 

 perature 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 

 heated body falls ; there is thus manifest- 

 ed a tendency to equalization of tempera- 

 ture, and such a state is finally reached, 

 after which no more change of tempera- 

 ture 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 ^. 

 sess this property in very different degrees. 



The experimental results on this subject 

 lead to no very definite conclusions. It 

 seems, however, that the porosity, or state 

 of division of the surface of a bod}^, has 

 the principal influence on its radiating 

 power. The less dense the surface, the 

 greater its radiating power. Radiation 

 seems to take place not merely from the 

 surface, but also from a little distance be- 

 neath it. 



2. Absorption of heat. — In our treatises 

 on natural philosophy, there is much ap- 

 parent confusion on this subject. Absorp' 

 tive power is often stated to be connected 

 with the color of a body. It is, however, the 

 fact that the radiating and absorptive pow- 

 er of a body for heat are absolutely equal. 

 That body which absorbs heat most readi- 

 ly, radiates it also most readily, and vice 

 versa. It must be understood, however, 

 that bodies may differ in their power of 

 absorbing or radiating heat of different de- 

 grees of intensity. Lampblack absorbs 

 and radiates heat of all intensities in the 

 same degree. White-lead absorbs heat of 

 low intensity (such as radiates from a ves- 

 sel filled with boiling water) as fully as 

 lampblack, but of the intense heat of a lamp 

 it absorbs only about one-half as much. — 

 Snow seems to resemble white-lead in this 

 respect. If a black cloth or black paper 

 be spread on the surface of snow, upon 

 w^hich the sun is shining, it will melt much 

 faster under the cloth than elsewhere, and 

 this too if the cloth be not in contact with, 

 but suspended above the snow\ In our 

 latitude every one has had opportunity to 

 observe that snow thaws most rapidly 

 when covered by or lying on black earth. 

 The reason is that snow absorbs heat of 

 low intensity with greatest facility. The 

 heat of the sun is converted from a high 

 to a low intensity, by being absorbed and 

 then radiated by the black material. But 

 ^ it is not color that determines this differ- 

 ence of absorptive power, for indigo and 

 Prussian blue though of nearly the same 

 : color, have very different absorptive pow- 

 'ers. So far, however, as our observations 

 .extend, it appears that dark-colored soils 

 j usually absorb heat more rapidly, and that 

 ^the sun's rays have least effect on light 



