EXPERIMENT STATION BULLETINS. 623 



and dry soil than in the soil of uniform moisture content, is due mainly 

 to the cheese-cloth which is placed between the dry and moist columns 

 of soil. Although this cheese-cloth was very thin and had large perfora- 

 tions, yet it prevented the two columns from forming a complete and per- 

 fect contact, and consequently the dry soil had to absorb water directly 

 through the cheese-cloth, as well as from the soil. 



Another factor which would seem to impede the rate of water move- 

 ment from moist and warm to dry and cold column of soil is the resist- 

 ance which the dry soils offer to wetting, due to the air film surround- 

 ing the particles and to any oily substance that might be present. The 

 influence of this factor, however, must be extremely small, if any, be- 

 cause when these soils were slightly damped the amount of water moved 

 was generally less or about the same as before. The common 'belief that 

 ivater moves more rapidly in damp than in dry soils is generally exaggerat- 

 ed. When a soil is damped to eliminate the factor of resistance to ivet- 

 ting its absorptive power for icater is decreased correspondingly so that 

 one factor tends to counterbalance the other, and at the end the results 

 are about tlie same. Moreover the soils which stubbornly resist wetting 

 are not very common. 



From the practical standpoint the results of the second part of the 

 present investigation are probably far more important than those of the 

 first part just disscussed. These results shoio the remarkable fact that 

 ivhen the dry soil is kept at 20° and JiO° G. and the moist soil at 0° 

 C. the dry soil takes up very little if any, ivater from the moist soil, and 

 that this quantity of water, absorbed decreases ivith rise in temperature. 

 As will be seen from the figures the percentage of moisture absorbed by 

 the dry soil at 20° C, is in all cases greater than that absorbed at 40° 

 C. At both amplitudes of temperature, the percentage taken up increases 

 with the colloidal content in the soil, which is natural. 



Obviously then, the temperature has a tremendous influence upon the 

 absorptive power of soils for water. This is what might be expected from 

 the laws of kinetic energy. According to this law the energy or motion 

 of the molecules increases with temperature, and consequently the ad- 

 hesive and absorptive forces of the solid matter for liquids or gases, de- 

 creases. These results then, tend to confirm postulate 2 page 14 that 

 the attractive forces of the soil for water decrease with rise in tempera- 

 ture. 



The foregoing experimental results and theoretical consideration sug- 

 gest very strongly that the efficiency of the soil mulches in conserving 

 moisture in the soil is not dependent solely upon their thickness and de- 

 gree of capillary discontinuity between themselves and the moist soil 

 beloiv, but also upon their temperature. It is well known that the tem- 

 perature of the surface soils during sun insolation is many degrees higher 

 than that of the air immediately above. In some parts of the world, 

 where the sky is clear and the sun insolation very intense, the surface 

 soil may attain a temperature about 40° 0. higher than that of the air 

 about 4 feet from the ground. Even at this Station the surface soil 

 temperature of the mineral soils and especially of the light sandy soils, 

 is very often approximately 15° C. higher than that of the air above. 

 From the surface downward the soil temperature decreases, but in the 

 upper 1 or 2 inches the diminution is far more rapid than at the lower 

 depths, amounting sometimes, and in certain soils, over 10° C. for each 



