506 BTATE BOARD OF AGRICULTURE. 



Now, the next question is how may this peculiar thermal translocation 

 of water be explained? What are the causal agents which bring it 

 about? 



As already stated it is not entirely due to the surface tension and vis- 

 cosity of the soil water; for if that Avere the case then tlie movement 

 should have followed a different course. If the soil exerted no adhesive 

 force for water than the amount of moisture moved from warm to cold 

 column of soil should be the same for all moisture contents, providing 

 the force of gravity is eliminated, for any particular amplitude of tem- 

 perature. But since the soil does exert a strong adhesive force for water 

 then the thermal motion of water should follow a straight line with rise 

 in moisture content, for any given difference in temperature. Instead, 

 the results plot into a parabola. Evidently, there must be another ex- 

 planation for the phenomena. 



The best explanation that is suggested appears to be founded upon the 

 following four assumptions: (l)The soil possesses an attractive power 

 for water and holds it with a great adhesive force; (2) these attractive 

 and adhesive forces decrease with increase in temperature; (3) the sur- 

 face tension or cohesive power of the liquid also diminishes with rise in 

 lemperature; and (4), the force due to the curvature of the water films 

 between the soil grains which are known as capillary films, decreases with 

 increase of Avater content. 



All Ihese four assumptions appear to be correct. The validity of the 

 third and fourth is generally recognized and consequently need no fur- 

 ther discussion. The validity of the first is also universally accepted; 

 that the soil possesses an attractive power for water can hardly be de- 

 nied; that the soil holds the water with a great adhesive force is evi- 

 denced by the great difficulty experienced in attempting to separate the 

 one from the other. Indeed, this adhesive force is so great that no 

 melhod as yet has been devised either to execute a complete separation 

 of the two components, or to measure with any degree of precision its 

 magnitude. The researches of Lagergren^, Young-, and Lord Kayleigh^ 

 indicate, however, that this force may be an order of magnitude from 

 G,000 to 25,000 atmospheres. As the water content increases this force 

 decreases. 



The great attractive and adhesive forces which the soil exerts for water 

 are further illustrated by the researches of Briggs and McLane on the 

 moisture equivalent and those of Briggs and Shantz* on the wilting co- 

 efficient of plants^ By whirling wetted soils in a rapidly revolving 

 centrifuge, fitted with a filtering device in the periphery, and developing 

 a force equivalent on the average to 3,000 times the attraction of gravity, 

 Briggs and McLane found that some clay soils would still contain about 

 50% of water. Biggs's and Shantz's studies on the wilting coefficient of 

 plants show that plants would wilt and die in clay soils even Avhen the 

 moisture content was still about 30%. 



Of all the four assumptions the correctness of the third, namely, that 

 the attractive and adhesive forces decrease with rise in temperature may 

 be doubted by many and challenged by few; the theoretical and experi- 



^Dber die beim Benetzen fein verteiller Korper auftretende WiirinetUnung von Lagergren, 

 Behang tUl K. SV. Ve-takakt., ITardl.. 24, afd, 11. No. 5, 1898. 



^Hydrostatics and elementary bjarukinetics. G. M. Minchin, p. 311, 1892. 



=0n the theory of surface forces, by Lord Rayleigb, Phil. Mag. (5), 30, 285-298, 456-475, 

 (1890). 



^Bulletin 45, U. S. Bureau of Soils. 



^Bulletin 230, U. S. Bureau of Plant Industry. 



