LECT. V.] THE ROOT. SOILS. 227 



which is the standard ; and, if it gain 165 grains, 

 it will be 1.825, water being 1.000*. A red or 

 yellow colour indicates the presence of iron ; and 

 the scratching glass when rubbed upon it, that of 

 silex. A given portion of the soil is next to be sub- 

 mitted to a degree of heat sufficient to dissipate 

 the whole of the water it contains without consum- 

 ing the vegetable and animal matter, or extricat- 

 ing the carbonic acid gas from the lime and other 

 calcareous substances it may contain. The tem- 

 perature should not exceed 300 Fahr. ; or a piece 

 of wood may be laid in the dish in which the pro- 

 cess is conducted, and whenever it " begins to be 

 " charred, the process must be stopped." The 

 specimen should now be again weighed, and when 

 in 400 grains the loss is 50, the soil may be re- 

 garded as highly absorbent and retentive of water, 

 and to contain either much decomposing organic 

 matter, or a large proportion of clay ; but when 

 the loss is under 20, it cannot be considered as 

 either very absorbent or very retentive, and is pro- 

 bably formed chiefly of sand. After bruising in a 

 mortar the portion thus treated, the larger stones, 

 gravel, and vegetable fibres should be separated 

 by the sieve, and their weight noted down. 



The sandy matter, which is insoluble, is easily 

 separated by boiling the specimen in three or 



* Davy's Elements of agricultural Ckymstry. 

 Q2 



