RELATIONS OF THE SOIL TO HEAT. 195 



ence between the absorption of radiant heat of low inten- 

 sity or its comumnication by conduction on one hand, and 

 that of high intensity like tlie Iieat of tlic sun on the other. 



Retention of Heat. — Other circumstances being equal, 

 the power of retaining heat (slowness of cooling) is th« 

 greater, the greater the weight of a given bulk of soil, 

 i. e., the larger and denser its particles. 



A soil covered with gravel cools much more slowly 

 than a sandy surface, and the lieat which it collects durhig a 

 sunny day it carries farther into the night ; hence gravelly 

 soils are adapted for such crops as are liable to fail of rip- 

 ening in cool situations, especially grapes, as has been 

 abundantly observed in practice. 



Color is without influence'on the loss of heat from the 

 soil by radiation, because the heat is of low intensity. 

 The porosity or rougliness of the surface (extent of sur- 

 face) determines cooling from this cause. Dew, which is 

 deposited as tlie result of cooling by radiation of lieat into 

 the sky, forms abundantly on grass and growing vege- 

 tation, and on vegetable mould, but is more rarely met with 

 on coarse sand or gravt'l. 



Influence of Moisture on the Temperature of the Soil. 

 — All soils, when thoroughly wet, seem to be nearly alike 

 in their power of absorbing and retaining Avarmth. This 

 is due to the fact that the capacity of water for heat is 

 much greater than that of the soil. We have seen that 

 lime sand and quartz sand are the sloAvest of all tlie in- 

 gredients of soils to suffer changes of temperature when 

 exposed to a given source of heat. (See table, p. 194.) 



Now, water is nine times slower than quartz in being 

 affected by changes of temperature, and as the entire sur- 

 face of the wet soil is water, whicli is, besides, a nearly 

 perfect non-conductor of heat, we can understand that ex- 

 ternal warmth must affect it slowly. 



Again, the immense co)tsum,ption of heat In. the forma- 

 tioiiof i^apor (see note, p. 188) must prevent the wet soil 



