[ 45 ] 



It is difficult to say where sandj begins and grit ends or where 

 stone ends and gravel begins. Sand may be again siliceous, 

 or micaceous, or calcareous, or felspathic. Stones and pebbles 

 are not immediately useful forlplant life but they serve useful 

 purpose in retaining moisture and acting as a reserve of plant 

 food. Stoney soils therefore though usually poor are not so 

 necessarily, and some stoney soils, viz,, those which contain 

 chiefly fossils, limestones, basaltic stones, and felspars, are 

 rkh. 



46. The mechanical analysis of soils is done by sifting and 

 washing. Sifting separates the coarser particles and wash- 

 ing the finer particles. The sample of soil to be analysed is 

 to be spread on the floor of a dry and warm room ; lumps are 

 to be broken up and crushed as drying proceeds The large 

 stones are then to be picked out, cleaned, dried and weighed. 

 The dry soil is then to be passed through a sieve the meshes 

 of which are 3 mm. in diameter. That which passes through 

 is weighed as fine earth, and what remains on the sieve as 

 gravel. The gravel is further washed and dried and weighed 

 again as true gravel. The fine earth is then boiled for an 

 hour to break up lumps, and it is then introduced into a 

 washing apparatus (e.g., Schulz's apparatus) in which by 

 introducing a flow of water at different rates, first the finest 

 suspended matter is washed]away^ and then successively the 

 finest sand and'coarser sand. 



47. Another process of mechanical analysis of soils 

 consists in arranging a series of vessels side by side and 

 allowing the water from the one to]flow into the next. This 

 also divides the soil into portions of different consistency. 

 For either process it is necessary finally to let the water 

 evaporate completely from each vessel of water and to weigh 

 the dry residue. This analysis enables us to separate the 

 soil into (i) stones ; (2) mechanical gravel ; (3) coarse sand ; 

 (4) fine sand ; (5) finest sand, and (6) clay and impalpable 

 matter. Clay soil proper is that which contains only clay 



