84 MOISTUKE LOSS ON IGNITION 



sieve is sometimes performed, depending upon its separation (i) by 

 sieves of suitable mesh, and (ii) by elutriation, i.e., washing in a current 

 of water of suitable strength and allowing the turbid liquid to settle. 



By the sieve method it is not advisable to attempt any further sub- 

 division than is given by holes 0*5 millimetre in diameter. Sieves of 

 3 millimetre, 1 millimetre, and 0'5 millimetre holes are sufficient for 

 most purposes, thus dividing the soil into four parts. If further 

 separation be desired, elutriation must be resorted to, for details of 

 which a treatise on soil analysis should be consulted. 



The usual sizes of particles into which a soil is divided in the 

 mechanical analysis, are 



(1) Stones and gravel, above 2 millimetres in diameter. 



(2) Coarse sand, between 2 and 0*2 millimetres in diameter. 



(3) Fine sand, between 0*2 and 0'02 millimetre in diameter. 



(4) Silt, between 0'02 and 0'002 millimetre in diameter. 



(5) Clay, less than 0'002 millimetre in diameter. 



Numbers 1 and 2 are usually separated by sieves [that for (2) hav- 

 ing about 80 meshes to the inch (linear)], the others by elutriation, the 

 times of subsidence, in a 10 centimetre column prescribed for the 

 separation, being l : 



Particles above '02 millimetres, 7J minutes. 

 -002 8 hours. 



In some cases a microscopical examination of the various sediments 

 is made in order to determine their mineralogical nature. Fora purely 

 chemical examination a sieve of 3 millimetre and one of 1 millimetre 

 perforations (practically i- in. and ^ in.) are all that are necessary. 



Chemical Analysis. A full chemical analysis of a soil, including 

 the determination of the quantities of every ingredient, is rarely re- 

 quired. If it be, the " fine soil " passing the 3 millimetre sieve is re- 

 duced to fine powder, the particles of which are, at least, able to pass 

 the 1 millimetre sieve. 



Determination of moisture. About 5 grammes of the air-dried fine 

 soil which has passed the 1 millimetre sieve are accurately weighed in 

 a flat-bottomed platinum dish previously weighed, with a short piece 

 of stout platinum wire to act as stirrer. The dish is heated to 100 in 

 a steam bath for 12 hours, with occasional stirring of its contents. It 

 is then cooled in a desiccator and weighed, again heated for an hour, 

 and re- weighed. If the difference between the two weighings does not 

 exceed 2 milligrams, the moisture is calculated from the loss in 

 weight. If the difference between the two weighings exceeds 2 milli- 

 grams the dish is again heated for an hour at 100 and re-weighed 

 until two successive weighings show less difference. 



Determination of loss on ignition. The residue from the moisture 

 determination is heated to low redness, with occasional stirring, until 

 all black particles are destroyed. This can very conveniently be done 

 in a muffle furnace. The dish is removed, allowed to cool, the con- 

 tents moistened with ammonium carbonate solution, dried, heated to 



, Fourth Eep. Gordon College, Khartoum, Chem. Sect., 1911, 34. 



