METHODS OF MECHANICAL ANALYSIS 13 



to the previous sediment ; this precaution is necessary, as the 

 first clay water may have been sufficiently strong to hinder 

 by its viscosity the subsidence of the coarser particles. 



We now have the soil divided into two portions, i. the 

 mixed sediments ; 2. the turbid water containing the finest 

 silt and clay. The turbid water is now allowed to stand in 

 a cylindrical vessel, 20 cm. in height, from twenty-four to sixty 

 hours, till all silt has deposited. The clay water is then decanted. 

 The sediment is rubbed with a caoutchouc-covered pestle, 

 well mixed with water, and the whole again allowed to stand 

 for twenty-four hours, and this treatment is repeated till the 

 sediment is free from clay ; it can then be dried and weighed. 

 This sediment corresponds with Group No. 1 2 in Table I. The 

 clay is then precipitated by adding 50 cc. of saturated brine 

 to each litre of clay water. The clay falls as a gelatinous 

 precipitate ; it is collected on a weighed filter, washed with 

 weak brine, dried and weighed. The filter containing the 

 dried clay is then replaced in the funnel, and well washed 

 with a weak solution of ammonium chloride. The washings 

 are evaporated to dryness, the residue heated to expel ammo- 

 nium chloride, and weighed. The combined weight of the 

 paper, and of the sodium chloride obtained in the last opera- 

 tion, is deducted from the previous weight of the filter and 

 contents, thus giving the weight of the clay. 



The mixed sediment, containing all except the finest con- 

 stituents of the sifted soil, is next introduced into Hilgard's 

 churn elutriator, and separated by currents of water of definite 

 velocity into nine groups. The construction of the elutriator 

 is shown in Fig. 2. The drawing is taken by permission 

 from Wiley's Agricultural Analysis. 



The elutriator consists of an upright glass cylinder, 300 mm. 



