202 



On the Physical Properties of Soil. 



endued with a very great power of containing water^ and have, 

 in the first place, to lose by evaporation a portion of this water 

 contained in them. 



4. The small power of retaining heat evinced by fine mag- 

 nesia, prepared artificially, would seldom be the same as that 

 which this earth would have as a mixed ingredient occurring natu- 

 rally in soils, being usually found under such circumstances in 

 a coarser form combined with other earths, as in sands and slaty 

 marls, which possess a great power of retaining heat. 



5. If we compare in the earths their power of retaining heat 

 with their other physical properties, we shall find it to be most 

 nearly in proportion to the weight of a determinate volume, that 

 is, to the absolute weight ; the greater mass an earth possesses in 

 the same volume, the greater will be in general its pov/er of re- 

 taining heat ; we may, therefore, from the absolute weight of an 

 earth, conclude, with a tolerable degree of probability, as to its 

 greater or less power of retaining heat. 



IX. Warming of Soils by the Sun. — The earths acquire heat 

 from the sun in different proportions, and this property may 

 exert a sensible influence on vegetation ; upon it, therefore, may 

 be founded, in some degree, the terms of a cold or hot soil. 

 Land consisting of a light- coloured clay is warmed much more 

 slowly and less powerfully by the sunlight, than one consisting of 

 a dark-coloured dry soil; black garden-mould, rich in humus^ 

 becomes much warmer than meagre limestone or clay soils. 



Very different external circumstances have an influence on the 

 degree of warmth thus imparted, and these may be referred to the 

 following four points : — 1. The different colour of the surface of 

 the earth ; 2, The different degree of dampness in which the earth 

 exposed to the sun's rays happens to be at the time ; 3. The 

 different component materials of the soil itself; and 4. The differ- 

 ent angle at which the rays of the sun fall upon the soil : the in- 

 fluence of each of these circumstances requires to be examined. 



Influence of the Colour of Soils on the warmth received by them 

 from the Sun. — The influence of colour on the amount of heat may 

 be observed easily in the following manner. We place thermo- 

 meters m the several soils, covering their bulbs about an eighth 

 of an inch high with earth ; in order to impart to each a different 

 colour, we sprinkle them over respectively with differently co- 

 loured powders, leaving one of them exposed to the sunlight in 

 its natural state and colour ; for the communication of a black 

 colour we may employ the soot obtained in the combustion of fir 

 and resin (lamp-black) ; and for a white colour, fine magnesia ; 

 these are to be sprinkled over the surface of the soils by means of 

 a fine lawn sieve 



