414 SECTIONAL TRANSACTIONS.— M. 



more complicated physical properties of the clays, such as plasticity, etc., 

 follows naturally. 



Then a group of mineralogical problems of some complexity presents 

 itself. The usual methods of identification and characterisation break down 

 when dealing with clay particles, and a new technique must be devised. 

 The density, refractive index, and double refraction can still be determined, 

 using modified methods. The fact that even the smallest particles of clays 

 show a high double refraction indicates that clays are essentially crystalline, 

 whilst the actual values of the double refraction serve to differentiate between 

 different clay minerals. The study of the double refraction of clays also 

 leads to the conclusion that where the base exchange capacity is high, a 

 large proportion of the cations concerned have definite positions in the 

 crystal lattice. 



Lastly come physico-chemical problems associated both with the minera- 

 logical character and the surface properties of the particles — e.g. coagulation 

 and base exchange. 



Dr. R. K. ScHOFiELD. — Water films in clay. 



Many of the properties which characterise clay in the agricultural and 

 general sense arise from a special relationship which exists between the solid 

 matter of the individual particles and the water which surrounds them and 

 links them together. The forces at work here are not capillary forces as 

 ordinarily understood, which arise from the pressure difference across 

 sharply curved menisci. Ordinary capillary forces could not give rise to 

 the swelling which takes place when a block of dry clay is moistened. 



If the uptake of water by dry clay is controlled by allowing it to come to 

 equilibrium with an unsaturated atmosphere of fixed relative humidity, it 

 is found that the amount taken up bears a close relationship to the base 

 exchange capacity. As base exchange must occur on ' active spots ' 

 distributed on (or possibly through) the clay particles, it would seem that 

 the water taken up from an unsaturated atmosphere is in some way associated 

 with these active spots. 



Under saturated conditions no direct connection exists between the water 

 content of the clay and its base exchange properties. Here the question as 

 to why the clay particles, which have been separated by the penetration of 

 water between them, should still cohere across this water film deserves 

 special attention. 



Discussion. 



Tuesday, September 6. 



Discussion on The distribution of agricultural products (Mr. E. M. H. 

 Lloyd; Mr. F. J. Prewett ; Mr. T. G. Henderson; Mr. J. M. 

 Caie ; Mr. C. S. Orwin) :— 



Mr. E. M. H. Lloyd. — Planning and world markets ; attempts to 

 regulate supplies in the marketing of primary products. 



Attempts to control economic forces by deliberate planning, particularly 

 in the distribution of agricultural products, have been stimulated by the 

 catastrophic fall of prices. Tariffs are supplemented by quotas or quantita- 

 tive restriction of imports. Prices may be raised by subsidies or by restric- 

 tion of production. Exports may be stimulated by bounties or limited by 

 monopolistic control. These measures may be statutory or voluntary, but 



