226 CONSERVATIVE ORGANS. [LECT. V. 



" rest. Thus, a sandy soil may sometimes owe its 

 " fertility to the power of the subsoil to retain 

 " water ; and an absorbent clayey soil may oc- 

 " casionally be prevented from being barren,, in a 

 " moist climate, by the influence of a substra- 

 " turn of sand or gravel." Notwithstanding these 

 obstacles, however, to the formation of perfectly 

 correct results in the chymical analysis of soils, 

 still, by observing the circumstances which may 

 thus alter the properties of a soil, and making 

 allowances for them, we are enabled by its means 

 to form a tolerably accurate notion of the com- 

 parative value of soils. 



When any soil is to be examined, the speci* 

 men, which should never be less than three or four 

 hundred grains, must be spread out to dry, and 

 then carefully weighed. As it is of importance 

 to discover the physical properties of a soil prior 

 to its analysis, for these direct in some respects 

 the experiments that may be necessary, the spe- 

 cific gravity, colour, and consistence of the spe- 

 cimen should be next ascertained. If a phial, 

 which holds 400 grains of water, be half filled 

 with that fluid and the soil introduced until the 

 liquid rises to the mouth, and then weighed, the 

 difference between the weight of the soil and that 

 of the water (which is known) will give the spe- 

 cific gravity of the soil. Thus, if the phial filled 

 as described gains 200 grains in weight, the gra- 

 vity of the soil will be 2, or double that of water 



