PRACTICE XXXIII 



MECHANICAL ANALYSIS 



Four samples of from 5 to 10 g. of the prepared soil ground 

 with a rubber pestle are weighed out and the hygroscopic mois- 

 ture determined. 



Two of these are then ignited and the per cent of loss on 

 ignition is found, based upon the water-free soil. Each of the 

 other two samples is placed in a shaker bottle and about 200 cc. 

 of distilled water and from 5 to 10 cc. of ammonia are added. 

 The bottles are then placed in the shaker and agitated until a 

 microscopic examination of a drop of the contents shows that 

 the soil particles are completely separated and no granules exist. 

 When this condition is reached 'the individual particles will ap- 

 pear clear or semitransparent in the field of the microscope, while 

 any remaining granules will be dark, irregular, and opaque. It 

 may be necessary to continue the shaking for 1 2 or even 24 hours 

 to 'completely disintegrate the soil granules. As the determination 

 is quantitative, only a small amount of the liquid is taken from 

 the bottle with a small glass tube and mounted on a slide for 

 examination. When the examination is completed, the slide and 

 cover glass are carefully rinsed with distilled water back into the 

 bottle to recover the small portion of soil taken. Great care is 

 necessary throughout the analysis to prevent the loss of any part 

 of the sample, and for purposes of comparison and greater accu- 

 racy in results duplicate samples are used. 



66 



