8 PHYSICAL CONSTITUTION OF SOIL 



scientific purposes to the uncertain names already mentioned. 

 The following classification of soil constituents given by 

 Wollny (Exp. Station Record, vi. 762) will afford a good idea 

 of the divisions now made in the more complete methods of 

 soil analysis. 



Diameter of Particles. 



1. Stones ... ..'. ... over 10 mm. 1 



2. Coarse Gravel 2 5 -10mm. 



3. Medium Gravel ... ... 2 -5 mm. 



4. Fine Gravel 1 - 2mm. 



5. Coarse Sand ... ... 0-5 - 1-0 mm. 



6. Medium Sand 0-25 - 0.5mm. 



7. Fine Sand 0-1 - 0.25mm. 



8. Coarse Silt 0-05 - 04 mm. 



9. Medium Silt 0-025 - 0-05 mm. 



10. Fine Silt 0-005 - 0-025 mm. 



11. Clay 0-0001- 0-005 mm. 



In a mechanical analysis the coarser groups of particles 

 are separated by means of metal sieves with circular holes ; 

 the smallest diameter of hole which can be practically used 

 is 0-5 or 0-25 mm. With the finer sieves the soil is used in 

 a wet state, and the work aided by a gentle stream of water. 

 The residue remaining on each sieve must be thoroughly 

 washed with water, and if necessary rubbed with a soft 

 brush, to separate any adhering finer matter. If the soil 

 contains much clay, the separation is greatly aided by 

 previously boiling the soil with distilled water for several 

 hours. Should however the coarse particles be composed of 

 slate, or other soft material, the boiling will disintegrate 



1 A millimetre is about ^ inch ; it is the most convenient unit of length 

 for small dimensions. 



9 The terms gravel, sand, &c., used for designating the physical constituents 

 of a soil are to be understood as expressing the size and not the nature of 

 these constituents. The sand, for instance, may consist of quartz, of felspar, 

 of limestone, or of a variety of other materials. 



