224 ANALYSIS OF HEAVY MINERAL SOIL. 



sieves employed. The percentages of these, and also of the smaller 

 fractions, are determined by weight. Now in the soils referred 

 to each mechanical fraction consisted to a considerable extent of 

 iron ores, chiefly magnetite. The specific gravity of the particles 

 separated by means of a magnet from two different soil samples 

 was found to be 4-6. The normal density of soil is approximately 

 2-6. It is obvious that a given number of magnetite particles of 

 any one size will influence the texture of the soil to exactly the 

 same extent as the same number of, say, quartz particles of the 

 same size, yet the weight of the former will stand to the weight 

 of the latter as 4-6 : 2-6. In other words the common basis of 

 comparison is removed and correlation in the ordinary way is of 

 no value. If the soil were homogeneous this difficulty would not 

 arise, since percentages by weight would then accurately represent 

 percentages by volume, but since soil is an extremely hetero- 

 geneous substance, and the soil which forms the subject of this 

 note particularly so as regards the relations between the weight 

 and the volume of the component particles, the question must be 

 given consideration. 



The finer particles are grouped according to size by elutriation 

 or sedimentation. The latter process only will be considered, but 

 the remarks made may be applied, suitably modified, to elutriation 

 processes. 



Hall,* whose method of mechanical analysis is used in the 

 laboratory of the Division of Chemistry, has shown that the limit- 

 ing diameters of particles separated by sedimentation are in agree- 

 ment with the expression, 



2g« 2 (o- — p) 



deduced from the work of Stokes for the velocity of a spherical 

 particle falling through a viscous medium. (v= the velocity of 

 the falling particle, a its radius taken as a sphere, <r its density, 

 P the density of the medium, and t] the coefficient of viscosity of 

 the medium.) From this expression we get for any given velocity 

 of a particle falling in water 



l o-- 



as the relationship between the limiting radius of the particle and 

 its density. Thus, the lrmiting size for magnetite particles of 

 density 4-6 is approximately two-thirds of that for quartz particles 

 of density 2-6. That is, each mechanical fraction in the case of a 

 soil containing the heavy mineral (magnetite) will contain par- 

 ticles of the heavy mineral of smaller size than the limiting or 

 smallest size for the corresponding fraction of normal soils. Micro- 



*Hall, A. D. (1904), "The Mechanical Analysis of Soils and the Composition 

 of the Fractions resulting therefrom." Jour. Chem. Soe., LXXXV, 950. 



