54 



value. Contact with water or water vapour breaks up soil aggre- 

 gates only very slowly. 



(d) A progressive decrease in the dispersion factor is caused 

 by heating the soil to temperatures over 110° C, but up to this 

 temperature no reduction appears. In the case of clay, heating 

 to 100° C. greatly reduces the dispersion factor. 



(e) The influence of electrolytes is progressive and gradual, 

 and not a sharp flocculation or deflocculation. With successive 

 increases in concentration of good deflocculants the dispersion 

 factor increases to a maximum, then decreases slowly, and then 

 rapidly until complete flocculation occurs. 



(/) A comparison of various methods recommended for soil 

 dispersion shows that the use of a rubber pestle is one of the 

 most efficient means. 



XXIV. A. N. Puri. " A Critical Study of the Hygroscopic 

 Coefficient of Svil." Journal of Agricultural Science, 

 1925. Vol. XV., pp. 272-283. 



The Hygroscopic Coefficient, defined as the percentage by 

 weight of water held by a soil when in equilibrium with an 

 atmosphere saturated with water vapour, has been much used, 

 especially in America, as a means of characterising a soil. Accu- 

 rate determination of the value is not easy, owing, among other 

 things, to the difficulty of maintaining a correctly saturated atmo- 

 sphere. The present investigations were made with a technique 

 deliberately refined beyond that possible in routine laboratory 

 determinations, in order to obtain some idea as to the inherent 

 value of the method itself, and of the justification of the con- 

 ception of the Hygroscopic Coefficient. 



The results, while incidentally clearing up the controversy 

 whether the Hygroscopic Coefficient, as determined under ordi- 

 nary conditions, increases or decreases with increase of tempera- 

 ture, show definitely that even with a very careful technique, 

 only qualitative accuracy can be obtained. The paper concludes 

 with a short discussion of the manner in which soil absorbs 

 water vapour, in which the conception of the Hygroscopic Co- 

 efficient is criticised on physical grounds. 



XXV. W. B. Haines. " Studies in the Physical Properties 

 of Soils. I. Mechanical Properties Concerned in 

 Cultivation/' Journal of Agricultural Science, 1925. 

 Vol. XV., pp. 178-200. 



The general problem considered in this paper is that of 

 supplying, by means of laboratory tests, data as to the mechanical 

 behaviour of soils sufficient to form a basis for the mathematical 

 treatment of ploughing and cultivation operations in the same 

 way that other engineering problems are usually treated. As a 

 first step to this end certain physical investigations already carried 

 out ;it Rothamsted have been grouped together under the follow- 

 ing heads : — 



(a) Soil cohesion. 



(b) Soil plasticity. 



(c) Friction between a metal surface and soil. 



