MUD — INORGANIC MANURES. 319 



sucli as whinstone, are more valuable than those from 

 grit or sandstone. 



866. This difference depends entirely on the alka- 

 line matters, &c., which the whinstone contains; but 

 besides this, road scrapings are often rich in animal 

 matters, derived from the dung and urine of cattle. 

 Road-drift is seldom used alone, but forms an excel- 

 lent addition to more concentrated manures, such as 

 alkaline salts, guano, super-phosphate of lime, &c. 



867. The fine mud deposited in stagnant waters, 

 and on the banks of many rivers, is of value as ma- 

 nure, though by no means so valuable a substance as 

 is frequently supposed ; its composition varies con- 

 siderably, and sometimes, particularly in the vicinity 

 of large towns, it contains a considerable quantity of 

 animal matter, phosphoric acid, and ammonia ; but, 

 generally speaking, the most valuable part of the 

 manure is carried away by the water (826). Even 

 in those cases when it is rich in animal matters, its 

 weight, and the heavy expense of carrying it to any 

 distance, very seriously interfere with its use. 



868. The mode in which many inorganic matters 

 act is very imperfectly understood, and must of neces- 

 sity remain so until the office performed by saline 

 and earthy substances in the nutrition of plants is 

 explained. It is, however, certain that the addition 

 of an earthy substance to the soil is of no use, if the 

 soil already contains abundance of that particular 

 compound ; and consequently that the applicability 



