114 The Petrography and Genesis of Sediments 



that is practically attainable. Theoretically there would seem, however, 

 possibilities of its unlimited extension to finer sizes in the centrifugal 

 elutriator of Yoder, 1 in which the velocity that the particles oppose to the 

 current is greatly increased by centrifugal force. 



The second method, developed by Mitscherlich, 2 determines the rela- 

 tive internal surface of a soil. There are two distinct procedures for 

 arriving at this quantity. The first is based on the fact that when water 

 is brought into contact with a perfectly dry porous or powdered substance 

 a certain heat is developed which is a function of the internal surface of 

 the substance. The finer its particles the greater, of course, will this 

 surface be, and the greater, therefore, the heat developed. In practice it 

 is more convenient to adopt the second procedure, which determines the 

 hygroscopicity of the substance, that is, the amount of water which the 

 material will take up out of a saturated atmosphere. This quantity, as 

 explained by Mitscherlich, is also supposed to have a definite relation to 

 the internal surface. 3 



The third method, and the one most generally employed, is that of 

 sieving the sands in conjunction with washing out the mud. It is to this 

 group that the method of Thoulet belongs, as well as that of Murray, the 

 U. S. Department of Agriculture, and others. To these methods are 

 added certain accessory procedures (the essential part of some less gen- 

 erally practised methods) such as treatment in heavy liquid, by the electro- 

 magnet, with acids, etc. 



There is no room for lengthy discussion of the relative values of these 

 three methods ; but for the purpose in hand the method by elutriation is 

 theoretically and practically the most satisfactory, 4 since it classifies the 



1 Yoder, P. A., Bull. No. 89, Utah Bxper. Sta., 1904. 



2 Mitscherlich, E. A., Bodenkunde, Berlin, 1905, pp. 49-70. 



3 An attempt to eliminate the effect of the internal surface of the particles 

 (that is minute fissures or pores in them) has been made by Franz Scheefer: 

 Eine Methode zur Bestimmung der ausseren Bodenoberflache. Dissert. Konigs- 

 berg. i. Pr., 1909. 



4 Thoulet, Precis d'analyse (op. cit), 65-67. Hilgard, E. W., Soils. 1906, 



pp. 90-93. Ries, H., Clays. 1906, pp. 113-115.. Andree, K., Ueber Sediment- 



bildung am Meeresboden (op. cit.), pp. 350, 351. 



