SPECIFIC HEAT OF SOILS 167 



heat of the same volume of water is reckoned as i-o. This 

 uniformity is largely due to the presence of two factors in 

 varying proportions. The coarse sand contains the smallest 

 proportion of hygroscopic water, but it also possesses the 

 greatest weight per cubic foot. The peat has by much the 

 smallest weight per cubic foot, but it also contains the 

 largest amount of hygroscopic water. In a naturally dry 

 condition a soil will thus have about one-third the specific 

 heat of water ; or three cubic feet of soil will be warmed by 

 the sun to the same degree as one cubic foot of water. Of 

 soils in this air- dry condition, peat will have the lowest 

 specific heat, and clay the highest. 



The commonest condition, however, in which soils are 

 met with is the condition which is found after rain has 

 ceased, and all excess of water has been removed by prolonged 

 percolation. If we calculate the specific heats of different 

 soils per unit of volume when in this moist, but fully drained 

 condition, we no longer find the uniformity belonging to the 

 air-dry state, the result is now chiefly determined by the 

 amount of water which the soil has retained. A coarse sand, 

 retaining but little water when drained, is now the soil most 

 easily warmed ; the fine sands, loams, silts, and clays show 

 a higher specific heat, the figure rising as the proportion of 

 water retained increases. A wet peat is now at the head 

 of the list, and its specific heat differs not greatly from that 

 of its own bulk of water. 



Thus, under natural conditions, the driest soil is the one 

 having the lowest specific heat, and therefore (other conditions 

 being equal) the one reaching the highest temperature when 

 exposed to solar radiation. We have already mentioned that 

 it is the soils with coarse particles, retaining little water when 



