1 48 Journal of Agricultural Research vo1.v,no.4 



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 were the case then the movement 

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

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

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

 force of gravity is eliminated for any particular amplitude of temperature. 

 But since the soil does exert a strong adhesive force for water, the thermal 

 motion of water should follow a straight line with rise in moisture con- 

 tent for any given difference in temperature. Instead, the results plot 

 into a parabola. Evidently there must be another explanation for the 

 phenomena. 



The best explanation suggested appears to be founded upon the follow- 

 ing four assumptions : (i) The soil possesses an attractive power for water 

 and holds it with a great adhesive force; (2) these attractive and adhe- 

 sive forces decrease with increase in temperature ; (3) the surface tension 

 or cohesive power of the liquid also diminishes with rise in temperature; 

 and (4) the force due to the curv^ature of the water films between the soil 

 grains, which are known as capillary films, decreases with increase of water 

 content. 



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

 third and fourth is generally recognized and consequently needs no further 

 discussion. The validity of the first is also universally accepted: That 

 the soil possesses an attractive power for water can hardly be denied; 

 that the soil holds the water with a great adhesive force is evidenced by 

 the great difficulty experienced in attempting to separate the one from 

 the other. Indeed, this adhesive force is so great that no method as yet 

 has been devised either to execute a complete separation of the two com- 

 ponents or to measure with any degree of precision its magnitude. The 

 researches of Lagergren (8),^ Young,^ and Lord Rayleigh (10) indicate, 

 however, that this force may be of an order of magnitude from 6,000 

 to 25,000 atmospheres. 



The great attractive and adhesive forces which the soil exerts upon 

 water are further illustrated by the researches of Briggs and McLane(3) 

 on the moisture equivalent and by those of Briggs and Shantz (4) on the 

 wilting coefficient of plants. By whirling wetted soils in a rapidly revolv- 

 ing 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 per cent of water. The studies of Briggs and McLane on the wilting 

 coefficient of plants show that plants would wilt and die in clay soils even 



1 Reference is made by number to " Literature cited," p. 172. 



2 Cited by Minchin, G. M. Hydrostatics and Elementary Hydrokinetics. p. 311, London, 1892. 



