On the Physical Properties of Soil. 



203 



When soils are^ under these circumstances^ exposed to the 

 heat of the sun, the black-coloured specimen always attains a 

 considerably higher temperature than the naturally grey-coloured, 

 and the latter again a higher one than the artificially white-coloured 

 earth ; the differences of temperature, in these cases, amounting 

 usually to many degrees. In experiments, which I made on this 

 subject, in the latter end of August, when the temperature of the 

 air in the shade was 77° F., that of the surface of black- coloured 

 sand rose from 77° to 123J-° F. ; that Vv-ith the natural colour to 

 1 12J° F. ; and that, on the contrary, with the white, to only 110° F. 

 That is to say, while the warmth of the white-coloured sand rose 

 33°^ that of the black-coloured sand rose 46J°, or almost one-half 

 more. The other earths exhibit corresponding differences. When 

 the differently-coloured earths are even exposed for hours to the 

 sun, they never attain the same degree of temperature, the lighter- 

 coloured earths always remaining considerably cooler, while the 

 black-coloured acquire the greatest degree of heat. 



Hence we see why the mere sprinkling of earth, ashes, or other 

 powders of a dark colour on snow, accelerates its melting ; and also 

 why the dark colouring applied to inside and outside walls, or the 

 naturally dark colour of many kinds of slate and slaty marl, has 

 the effect of accelerating the ripening of fruit, as grapes, melons, 

 6cc., planted against them. 



Influence of Moisture on the Warming of Soils. — The influence 

 of the damp or dry state of soils on their acquisition of warmth is 

 also considerable. If we expose earths of the same kind in a dry 

 and wet state to the sun, the wet earth never attains the same de- 

 gree of heat ; its temperature, as long as it remains moist, being 

 always many degrees less than it would acquire in a dry state. 

 The depression of temperature arising from the evaporation of 

 their water, amounts to 11J° or 13J° F. 



As long as the several earths, at the early part of the experi- 

 ment, remain saturated with water, they exhibit but little differ- 

 ence in their powder of acquiring heat, as they give off to the air, 

 in this state of saturation with water, nearly equal quantities of 

 vapour, in the same time ; when they have become, how ever, in 

 some measure dried in the air, their differences of temperature 

 are found to become greater ; light -coloured earths, with great 

 powers of containing water, acquire heat in consequence the most 

 slowly, while dark- coloured sand and slates, on the contrary, with 

 less powers of containing water, become warm on both these ac- 

 counts, in a quicker and more pow^erful manner. 



Influence of the different Materials constituting Soil, on its ac- 

 quisition of Heat. — The different ingredients which enter into the 

 composition of soils have, in themselves^ far less influence on 



