SILICA: SA'ND'k'M ClAY 



lOI 



The Bohemian glass used for chemical apparatus consists of silicates of 

 potassium and calcium, window or crown glass of silicates of sodium 

 and calcium, flint or crystal glass of silicates of potassium and lead, and 

 bottle glass of silicates of sodium and calcium and some iron and aluminium. 

 Glass containing sodium or lead is "soft" and easily fusible, that contain- 

 ing potassium and calcium is "hard " and infusible. 



The Soil and its " Mechanical " Properties.— Soil essentially consists 

 cf sand and clay in varying proportions, mixed with more or less gravel, 

 chalk, and decaying vetegable matter ("humus"). The "mechanical 

 analysis," i.e. the determination of the pro- 

 portion of gravel, sand, and clay, may be 

 roughly carried out as follows :— From a 

 known weight of air-dried soil, sift out the 

 gravel and weigh it. Mix the sifted soil 

 thoroughly with water, and pour the mixture 

 into a wide tall cylinder (Fig. 40). Now 

 allow a very slow, steady stream of water to 

 run into the bottom of the cylinder, gently 

 stirring the contents from time to time with 

 the tube through which the water is passing, 

 and so arrange the supply of water that the 

 clay and very fine sand are washed away. 

 When, thoroughly washed, dry the residue and 

 weigh it. The quantity of clay and fine sand 

 is determined by difference. Express the 

 result as percentage of (i) gravel, (2) coarse 

 sand, (3) clay and fine sand. Upon the 

 proportion of sand and clay mainly depend 

 the dryness or wetness of a soil. This is 

 due, firstly, to the greater porosity of sand, 

 and secondly, to the greater capillarity of clay. 



If sandy and clayey soils be percolated with water (Fig. 16), the water 

 will pass through the sandy soil more quickly, because the particles are 

 coarser, and the spaces between therefore larger. The sandy soil is, in 

 fact, more porous. The gelatinous nature of the hydrated silicate of 

 alumina constituting clay, also renders clayey soils less porous. If such 

 a soil be "burnt," the anhydrous silicate is produced, and the soil is 

 rendered more porous. The same effect will be produced by the application 

 of lime, which renders the gelatinous clay more granular. (See above.) 



If two long glass tubes, filled wiih the two soils, previously thoroughly 

 dried and disintegrated, be allowed to stand in vessels of water (Fig. 41), 



Fig. 40. 



Fig. 41. 



