53 



definite relative humidities is almost independent of temperature 

 over the range 20° to 40° for high relative humidities, but 

 decreases markedly with inci easing- temperatures for the lower 

 relative humidities. This influence of temperature on the relative 

 vapour pressures of moist soils is connected with the fact that 

 dry soils liberate heat when wetted. All soils show considerable 

 hysteresis in their vapour pressure relationships. The apparent 

 water content or loss on heating of a soil increases regularly with 

 the temperature of heating up to about 200°C. Soils heated to 

 various temperatures between 100° and 200° C. show substantially 

 the same water absorptions at different relative humidities. The 

 water absorption by a soil is markedly affected by previous treat- 

 ment with agents known to disintegrate the soil. 



The vapour pressure curves of the various soil fractions, 

 including clay, differ only slightly in type from that of the soil, 

 although the absolute amounts of water taken up increase with 

 the increasing specific surfaces. 



Some preliminary data are given to show the complicated 

 effects resulting from addition of salts to the soil. 



XXIII. A. N. Puri and B. A. Keen. " The Dispersion of 

 Soil in Water under Various Conditions." Journal 

 of Agricultural Science, 1925. Vol. XV., pp. 147-161. 



A study has been made of the intensity of the forces binding 

 soil particles together, when the soil has been previously subjected 

 to treatments simulating various field conditions, and certain 

 laboratory processes connected with physical, chemical and bio- 

 logical investigations. 



The technique consisted in shaking soil with water under 

 reproducible conditions, allowing the mixture to stand for 24 

 hours, and then determining the concentration of soil in the top 

 8.5 cms. of the suspension : this was expressed as a percentage 

 of the original concentration, and the value thus obtained was 

 called the dispersion factor of the soil under the conditions of 

 treatment. 



The following conclusions emerge from the data : — 



(a) Disintegration of soil aggregates by shaking in water 

 proceeds continuously, rapidly at first and then more slowly. 

 After nearly 100 hours of shaking, the dispersion factor is still 

 slowly increasing, and its change with time after completion of 

 the first rapid increase can be expressed by the equation : — 



d = a + K log t. 

 where d = disperson factor, t = time of shaking, a and K = constants 



(b) The dispersion factor depends on the original concentra- 

 tion of the soil. There are slight but systematic changes in the 

 lower concentrations and flocculation occurs when a certain max- 

 imum concentration is passed. It is probable that, besides the 

 increase in concentration, the concomitant increase in the amount 

 of soluble salts present is concerned in the flocculation process. 



(c) The dispersion factor for clay decreases continuously with 

 decrease in initial moisture content, whereas with soil a stationary 

 value is reached when the moisture content is reduced to a certain 



