Studies in Soil Physics, IV. 169 



expected to ensue would be attained at the cost of an additional 

 operation (and a very troublesome one) with a consequent in- 

 crease in time and labor expended. The error could be made 

 insignificant only by the employment of a very large number 

 of groups and the determination of the amounts of material 

 lying within each. Obviously, such a large number of groups 

 means a multiplicity of analytical operations and an expensive 

 and laborous method. Any system of analysis with few enough 

 groups to be ])racticable is going to give at best onh a somewhat 

 rough approximation of the real mechanical com]:»osition. Fur- 

 thermore, in the place where the greatest practical errors occur, 

 namely in the finer particles, our laboratory methods for the 

 separations of are hardly accurate enough for any greatlv 

 closer spacing of the limits and these methods are probably 

 not susceptible of much improvement. Our present svstems 

 of mechanical analysis are about as good as we can hope to 

 get.* 



There is still a third major error in the mechanical analysis, 

 that introduced by the variations in the shapes of the soil par- 

 ticles. Shape and degree of roughness have various effects upon 

 the physical properties and still other and unrelated effects 

 upon the mechanical analysis. Indeed, since some of the 

 group separations are made by sieving, and some bv water 

 elutriation, the shape effects will be different on the different 

 groups of the same sample. Ordinarily, perhaps, these shape 

 effects are not particularly important .since soil particles are 

 usually roughly spherical in form and of about the same shape- 

 character in all ST^ils. But in special cases the error is by no 

 ne,.;ligible. A very micaceous soil, for instance, has very 

 different jdiysical properties from one of the same mechanical 

 anahsis bnt composed of particles which are mainlv spheres 

 insteads cf disks. 



These three errors of the mechanical analysis are all incor- 

 rigible. They may be denoted as (1) disunity of expression; 

 (2) failure to express conditions A\ithin the limits of the indi- 

 vidual groups; and (3) failure to take account of variations in 

 the shapes of the particles. All are inhe ent in the nature f f 

 the process and if we are to use at all tlie mechanical analysis 



♦Unless air elutriation for clays can be rendersd practicable, and this seems very 

 unlikely . 



