THE PHYSICAL PROPERTIES OP THE SOIL. 955 



The absorptive capacity and emission capacity of the soil for heat rays. — 

 No experiments are recorded regarding the absorptive capacity of tlie 

 soil for the true heat rays (invisible ultra-red) of the solar spectrum, 

 but the absorption of the visible rays depends on the color of the soil. 

 Assuming the behavior of the dark heat rays to be the same as that 

 of the luminous rays, we can say that the darker the color the more 

 the soil is warmed. The absorption of the luminous rays is of impor- 

 tance, because the proportion between luminous and non-luminous rays is 

 nearly constant in sunlight, the invisible radiation amounting to only 

 twice the visible radiation. 



The color of the soil tells us nothing concerning the emission capacity 

 for heat rays. Kirchhoff's law states that the ratio of emission to 

 absorption is the same in all bodies, but only for rays of the same wave 

 length at the same temperature. Soils when exposed to the sun absorb a 

 mixture of rays of every degree of fraugibility and temperature, but 

 emit only rays of slight refrangibility (dark rays) and low temperature; 

 therefore the rays absorbed are different from those emitted. From this 

 it follows that emission can not be directly estimated from absorption. 

 So far as we can generalize the results of experiments thus far made 

 along this line, it may be said that the mineral constituents radiate 

 heat better than the organic, and that of the former quartz radiates 

 most perfectly. In natural soils, which generally consist of a mixture 

 of constituents, the differences in radiation capacity are very small. 

 Water radiates heat rays to a greater extent than any of the solid co»m- 

 stitutents of the soil. Therefore when much moisture is present the 

 differences which would otherwise occur in the emission of heat disap- 

 pear almost entirely.^ 



The heat absorbed in evaporation at the surface of the soil. — The heat 

 of the sun absorbed by the soil is lost partly by being communicated 

 to and warming the lower strata of soil, and partly by being rendered 

 latent by evaporation. Loss of heat in the second way increases and 

 decreases, other things being equal, with the quantity of evaporation. 



The heat capacity [specific heat) of the soil. — This property, like the 

 water capacity, is most conveniently referred to the volume, and is 

 expressed by the number of units of heat necessary to raise the tem- 

 perature of a definite volume of the soil 1°, compared with the number 

 required to raise the same volume of water the same interval of tem- 

 perature, this number in the case of water being considered as unity. 

 The specific heat of the soil in the dry state amounts to one sixth to 

 one third that of water. Of the various soil constituents, quartz has 

 the highest specific heat (0.2919), humus the lowest (0.1647), while clay 

 is intermediate (0.2333). With an increase of water in the soil, the 

 specific heat of course increases; therefore in one and the same soil the 



*A. von Liebenberg, Untersuchungen iiber Bodenwarme; Habilitationsacbrift, 

 Halle, 1875. C. Lang, Forscb. Geb. agr. Pbys., 1, p. 379. E. Wollny, Ibid., p. 43; 4, 

 p. 327. J. Abr, Ibid., 8, p. 397. 

 18055— No. 11 2 



