KELATIONS OF TIIK SOIL TO HEAT. 1*^9 



the surface of the earth radiates into the cooler atmos- 

 phere aiid planetary sj^acc; the temperature of the surface 

 rapidly diminishes, and the air itself becomes cooler by 

 convection.* As the cooling goes on, the vapor suspend- 

 ed in the atmosphere begins to condense upon cool objects, 

 while its latent heat becoming free hinders the too sudden 

 reduction of temperature. The condensed water collects 

 ill drops — it is dew ; or hi the colder seasons it crystallizes 

 as hoar-frost. 



The deposition of liquid water takes place not on the 

 surface of the soil merely, but within it, and to that depth 

 in which the temperature falls during the night, viz., 12 

 to 18 inches. (Krutzsch observed the temperature of a 

 garden soil at the depth of one foot, to rise 3° F. on a 

 May day, from 9 A. M. to 7 P. M.) 



Since the air contained in the interstices of the soil is at 

 a little di'pth saturated with aqueous vapor, it results that 

 the slightest reduction of temper;iture must at once occa- 

 sion a deposition of water, so that the soil is thus supplied 

 with moisture independently of its hygroscopic power. 



Conditions tliat Affect the Temperature of the Soil.— 

 The special nature of the soil is closely connected with 

 the maintenance of a uniform temperature, with the pre- 

 vention of too great heat by day and cold by night, and 

 with the watering of vegetation by means of dew. It is, 

 however, in many cases only for a little space after seed- 

 time that the soil is greatly concerned in these jjrocesses. 

 So soon as it becomes covered with vegetation, the char- 



* Thoagh liquids and gases are almost perfect non-coiuliictors of heat, yet it can 

 (Bffuse through them rapidly, if advantage be taken of the fact that by lieatiiig they' 

 expand and therefore become specifically lighter. If heat be applied to the upper 

 surface of liquids or gases, they remain for a long time nearly unaflected ; if 

 it be applied beneath them, the lower layers of jjarticles become heated and rise, 

 tlieir place is supplied by others, and so currents upward and downward are 

 established, whereby the heat is rapidly and uniformly distributed. This process 

 of convection can rarely have any influence in tlie soil. What we have stated 

 concerning ft shows, however, in what way the atmosphere may coustantly act 

 in removin'j heat from the surface of the soil. 



