198 § 107. ANALYSIS OF SOILS AND ROCKS. 



centre of the box has reached a given point, say 70"^ or 

 80° C. 



3. The power of the soil to retain heat may be de- 

 termined by exposing the box of heated soil, obtained in 

 either of the preceding experiments, to the common tem- 

 perature of the air in the shade, and observing how long 

 a lime is required for the soil in the middle of the box to 

 cool to the temperature of tlie air, or to a given point, as 

 20° or 25° C. 



The behavior of the soil with respect to the heat of the 

 sun and of the atmosphere is of great agricultural im- 

 portance, and should be more carefully examined than 

 has hitherto been the case, by the careful performance of 

 experiments like those described above, and by series of 

 observations on the temperature of the soil in the field at 

 various depths, ranging from 3 cm. down to one metre or 

 more. 



d. The Specific Gravity of the Soil. — 1. This may 

 be determined in the usual manner for a powder (§ 35, V). 



A soil rich in humus is specifically the lightest, and 

 coarse sandy soils are the heaviest. 



2. The absolute weight of the soil is determined by 

 filling a glass vessel, or a cubical zinc box, whose weight 

 and capacity are known, with it, tapping the vessel occa- 

 sionally on the table while filling it, and weighing it. The 

 weight of a cubic metre or a cubic foot can then be cal- 

 culated from the result; the apparent specific gravity 

 can be estimated by the ratio between the weight of this 

 volume of soil and that of an equal volume of water. 



3. The apparent specific gravity of the soil dried at 

 100° C. may be estimated by subtracting the volume of 



. the water contained in the quantity of air-dried soil that 

 was weighed in this experiment from the volume of the 

 soil, and then a volume of water equal to the remainder is 

 taken for the divisor, and the weight of the soil dried at 

 100° for the dividend. 



