SCIENTIFIC AGRICULTURE. 159 



extensively used as a mordant in calico printing. Alumina is 

 supposed to contribute but little to the nourishment of plants: 

 it is said by Liebig to be an absorbent of ammonia: this, 

 however, is doubted by Prof. Johnston. Its principal agency 

 as ji.i element of soils, is of a mechanical kind. The salts of 

 alumina are few; they have not been sufficiently tested as 

 fertilizers to determine precisely their value in this respect: 

 Sprengol considers them highly deserving of trial in practical 

 agriculture. 



MANGANESE, 



This mjtal is diffused widely through nature, although not 

 in great abundance: it is found mostly in the mineral, but 

 traces of it also exist in the animal and vegetable kingdoms. 

 It has a very strong affinity for oxygen, and is therefore with 

 difficulty reduced from its oxides and ores, which, however, 

 may be done by a long continued and intense heat 



It is hard, brittle, granular, grayish white, and has a specific 

 gravity of 8 : it is very infusible, soon tarnishes by the absorp- 

 tion of oxygon, and after a while falls into a black powder. 

 There are several oxides, two acids, and many salts of manga- 

 nese, some of which are soluble and others insoluble in water. 

 It is used in the arts, and is probably a necessary ingredient 

 in soils. Its equivalent is 27.G7. 



IRON. 



This is the most important of all the metals, and is the most 

 extensively distributed over the earth. It is sometimes found 

 in loose blocks of pure metal on the surface ; but mostly in 

 veins and mines, combined with sulphur, forming a gold 

 colored ore, called sulphuret of iron, and with oxygen in the 

 form of the black and red oxides : it is also extensively com- 

 bined with carbonic acid, constituting the clay iron ore. Native 

 arsenites, phosphates, sulphates and other salts have been, 

 found. 



Nearly all reddish soils and stones are colored by oxide of 



