APPENDIX I 355 



beaker and made up to the 7*5 cm. mark. After stirring, it is allowed 

 to settle for twelve and a half minutes, and the liquid decanted off. 

 The operation is then repeated as before till all the sediment sinks 

 in twelve and a half minutes, leaving the liquid quite clear. The 

 sediment obtained is the " silt," which is dried and weighed as usual. 

 The liquid contains the " fine silt," which, when it has settled down, 

 can be separated by decanting off the clear liquid, and dried and 

 weighed. 



For silt minimum value of v = 001 cm. per second, minimum 

 diameter of particles = 0*012 mm. For fine silt the diameter ob- 

 viously lies between this value and the one found for clay. 



When it is desired to compare the results with American data 

 the fine silt can be divided into two groups ; settling for two hours 

 five minutes brings out a group o'oi to 0*005 mm. diameter, and 

 the remainder lies between 0*005 and 0*002 mm. diameter. The 

 fractions can then be made to correspond fairly closely with those 

 adopted in the United States. 



(6) Determinations are made of the "moisture" and "loss on 

 ignition " of another 10 grms. of the air-dry earth. The sum of the 

 weights of the fractions after ignition + loss on ignition + moisture 

 + material dissolved in weak acid should approximate to 10 grms. 



(7) It is advisable to make a control determination of the "fine 

 gravel" in a portion of fifty grms. of the air-dry earth. The soil 

 should be treated with acid, as in (i), and after that is removed by 

 decantation may be at once treated with dilute ammonia and washed 

 on the sieve with i mm. round holes. The " fine gravel " left on 

 the sieve is then dried and weighed. 



The American method is somewhat different. The breaking 

 down of the aggregates is brought about by physical means — e.g. 

 violent shaking — and sedimentation is hastened by a centrifugal 

 apparatus which is a distinct improvement on the British method : 

 the centrifuge was also used by Beam in Khartoum.^ Hilgard does not 

 adopt a sedimentation method but proceeds in the converse manner; 

 he collects and weighs the particles carried off by successive streams 

 of water of varying velocity. Full details are given in Bull. 24, 

 Bureau of Soils, 1904, and in Wiley's Agricultural Analysis, vol. i., 

 where the continental methods are also described. An entirely new 

 method has been described by Oden (218^). 



^ W. Beam, The Mechanical Analysis of Arid Soils (Cairo Scientific yournal, 

 igii, 5, 107-119). Pub. Wellcome Tropical Research Lab,, Khartoum. 



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