86 MECHANICAL ANAL VSIS OF SOIL 



the 8-5 cm. mark. The whole is then left to stand for 

 twenty-four hours. The effect of the ammonia is to 

 keep the clay particles in suspension, the heavier portions 

 settling to the bottom. The turbid supernatant liquid 

 is now rapidly poured off into a large jar or labelled 

 Winchester bottle, and the residue again stirred with 

 ammonia, water added to the 8-5 cm. mark, and allowed 

 to stand for twenty-four hours. The operation is 

 repeated until the liquid pours off quite clear, all the 

 washings being kept together. The united washings 

 are well shaken, their total volume measured, and 100 

 c.c. extracted and evaporated to dryness in a weighed 

 dish ; the residue is ignited over a Bunsen burner, 

 cooled, and weighed. From the weight, calculated 

 back on the whole of the washings, is obtained first 

 the weight and then the percentage of " clay " in the 

 soil. 



{c) The sediment left after removal of the " clay " is 

 again stirred well with ammonia waten which is now 

 added to a depth of 7-5 cm. The contents of the beaker 

 are allowed to stand for twelve and a half minutes, and 

 the liquid poured off into a Winchester as before. 

 This operation is repeated until the washings are quite 

 clear, after twelve and a half minutes' sedimentation. 

 The contents of the liquid are determined by evapora- 

 tion and ignition, as before, and the quantity gives the 

 proportion of ""^ fine silL" 



{d) The residue from {c) is again treated with 

 ammonia, as in the previous operations, the liquid being 

 filled up to the 10 cm. mark and the time of sedimenta- 

 tion reduced to one hundred seconds. When the liquid 

 comes off perfectly clear the residue is washed into a 

 porcelain dish, dried and ignited, and weighed as '^fine 

 sandy The washings from this give on evaporation and 

 ignition the fraction known as " silt." 



