MECHANICAL ANALYSIS. 91 



a rough mechanical analysis before commencing the chemical 

 analysis. 



Mechanical Analysis. The soil collected in its damp state 

 is first air-dried. This is done by spreading it on paper and 

 exposing to air for some days, care being taken to exclude 

 dust, products of combustion or other foreign matters. In 

 order to increase the rate of drying, the author has used a 

 glass chamber provided with shelves, from which air is drawn 

 by a fan, driven by an electric motor, the necessary supply of 

 fresh air being admitted at the bottom, after being warmed by 

 passing over resistance coils (used for diminishing the current 

 supplied to the motor) and filtered through calico. In this 

 way air at a temperature of 25 to 30 C. is drawn over the 

 soil and drying is completed in a short time. Before the soil 

 is thoroughly dry it is rubbed between the hands or with a 

 wooden or rubber-covered pestle, in order to crumble it to 

 powder, care being taken not to crush stones or mineral frag- 

 ments. When dry a convenient quantity, 1 or 2 kilograms, is 

 weighed out and passed through a sieve having meshes 3mm. 

 or |in. diameter (best made of metal with circular perfora- 

 tions). The portion failing to pass this is subjected again to 

 hand rubbing until no aggregations of clay, &c., are left. 

 Finally, the stones retained by the 3mm. sieve are removed 

 and weighed, the portion passing through is stored in a stop- 

 pered bottle for the chemical analysis and duly labelled, the 

 percentage of stones removed being conveniently recorded on 

 the label. 



It is rarely desirable to make an analysis of the stones, 

 though in a thorough examination of a soil their character 

 and probable composition should be recorded. 



Further mechanical analysis of the portion passing the 

 3mm. sieve is sometimes performed, depending upon its sepa- 

 ration (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 -5mm. in diameter. 

 Sieves of 3mm., 1mm., and -5mm. holes are sufficient for most 



