52 



known empirical expression : — 4.3 x (Hygroscopic Coefficient) 

 + 21. The present paper shows that the derivation of the value 

 23.46 per cent, cannot be substantiated, because the analyses 

 take no account of the fact that the adjacent water wedges sur- 

 rounding each point of contact of the spheres come into contact 

 with each other at a moisture content very considerably below 

 23.46 per cent. The value is in fact much too high, and Briggs' 

 figure 21 still remains empirical. An explanation is suggested 

 for the gradual decrease with height in the moisture content of 

 a long unbroken soil column saturated at the base. 



XXI. E. M. Crowther and A. N. Puri. " The Indirect 

 Measurement of the Aqueous Vapour-pressure of 

 Capillary Systems by the Freezing-point Depression of 

 Benzene." Proceedings of the Royal Society. A., 

 1924. Vol. CVL, pp. 232-242. 



With a view to developing a technique for the measurement 

 of vapour pressures in relatively dry soil, a study was made of 

 the freezing-point depressions (F.P.D.) of moist benzene in equili- 

 brium with the soil. Sidgwick's assumed proportionality between 

 the F.P.D. of benzene and the aqueous vapour pressure of the 

 with the soil. Sidgwick's assumed proportionality between the 

 F.P.D. of benzene and the aqueous vapour pressure of the 

 material with which it is in equilibrium, was substantiated by 

 experiments on sulphuric acid-water mixtures. All soils showed 

 a systematic deviation, the observed F.P.D. being in all cases 

 greater than that calculated from the vapour pressure. By 

 postulating a system of micropores or capillaries in the soil, and 

 allowing for the effect of benzene on the surface tension of the 

 soil water, an expression was obtained which agreed with the 

 observed values. This agreement supports the view that many 

 of the observed colloidal properties of soils can be interpreted 

 in terms of minute capillaries. 



XXII. A. N. Puri, E. M. Crowther and B. A. Keen. 

 " The Relation between the Vapour Pressure and 

 Water Content of Soils/' Journal of Agricultural 

 Science, 1925. Vol. XV., pp. 68-88. 



Much of the modern work on the physical properties of soils 

 has been interpreted on a colloidal basis. There is evidence that 

 the colloidal portion can be regarded as possessing a reticulate 

 structure, possibly analogous to that shown to exist in silica gels. 

 These minute pores largely control the vapour pressure of soils 

 at different moisture contents, and a measurement of this property 

 offers a promising line of attack on the physical relations between 

 the colloidal soil material and water. 



Three experimental methods were tried and the most con- 

 venient was one in which the soils were allowed to come into 

 equilibrium in a vacuum dessicator, over sulphuric acid of the 

 desired strength, Some of the soils were subjected to various 

 treatment* known to affect other physical properties, such as 

 ^sive wetting and drying, heating, and addition of salts. 

 I he general results were as follows : — The water absorption at 



