ii.] MECHANICAL ANALYSIS 55 



When the samples reach the laboratory they are 

 spread out on shallow trays to dry, which process may 

 be accelerated by a gentle warmth, not exceeding 40 C. 

 In dealing with stiff soils it is advisable to crumble all 

 the lumps by hand while the earth is still somewhat 

 moist. When the whole is sensibly dry the stones are 

 separated by a sieve having round holes 3 mm. in 

 diameter; the material that does not pass the sieve 

 is gently worked up in a mortar with a wooden pestle, 

 care being taken not to break the stones, chalk, etc., 

 but only to crush the lumps of earth. Finally, the 

 material upon the sieve is roughly weighed and well 

 washed in a stream of water till all the fine earth is 

 gone, dried, picked over to free it from roots and 

 stubble, and weighed as " stones." To get the pro- 

 portion borne by the stones to the soil, the fine earth 

 is also weighed, an addition being made of the weight 

 lost by the stones in washing. 



Of course the figure obtained for the proportion of 

 stones is only approximate, for if the stones are of any 

 size they will be very irregularly caught by the auger 

 or even by the 6-inch square tool. The material passing 

 the sieve is again spread out in a thin layer in an 

 ordinary room, until the surface maintains the same 

 colour as the lower layers ; it is then bottled up as 

 "air-dry fine earth" for analysis. 



The Mechanical Analysis of a Soil. 



The mechanical analysis that follows consists in 

 dividing the fine earth into a series of fractions con- 

 sisting of particles of known size ; we can use sieves to 

 sort out the coarser grades, but the finer ones must be 

 separated by their relative powers of remaining sus- 

 pended in water. 



The methods in use depend on two principles : 



