52 THE MECHANICAL ANALYSIS OF SOILS [chap. 



The portion which remains on the sieve is dried and 

 weighed. It is then divided into "fine gravel" and 

 " coarse sand " by means of a sieve with round holes of 

 I mm. in diameter, the portion retained by the sieve 

 being designated " fine gravel." 



3. The portion in the beaker is well worked up with 

 the rubber pestle, ammoniacal water is added up to the 

 8- 5 cm. mark, and the whole is put aside to stand for 

 twenty-four hours. The turbid, supernatant liquid is 

 then rapidly poured off into a large jar, and the residue 

 is rubbed up again with the rubber pestle and more 

 ammoniacal water, as before. The whole operation of 

 filling to the mark, standing for twenty-four hours, and 

 pouring off the turbid liquid is carried through as 

 before, and repeated as long as any matter remains 

 in suspension for twenty-four hours. Generally seven 

 to ten decantations will be sufficient, after which the 

 united turbid liquid is evaporated to dryness in a tared 

 basin, and weighed. This fraction consists of the "clay" 

 particles less than 0002 mm. in diameter, together with 

 all the soluble and some of the insoluble humus. The 

 contents of the dish are ignited over an Argand burner 

 for some time and reweighed, to obtain the weight of the 

 " clay " after ignition. 



4. The sediment from which the clay has been 

 removed is worked up as before in the beaker, which, 

 however, is now only filled to the depth of 75 cm. 

 The contents are now allowed to stand for twelve 

 and a half minutes only, when the liquid is poured 

 off into a large jar as before. The operations 

 are then repeated until all the sediment settles 

 in twelve and a half minutes and the liquid above 

 is left quite clear. The contents of the jar are now 

 evaporated to dryness and weighed, as in operation 

 3, before and after ignition ; this fraction is desig- 



