24 SCIENTIFIC AGRICULTURE. 



used for lighting up caves (24). Aluminium has 

 been used to some extent for ornamental and other 

 purposes, but it is so difficult and expensive to sep- 

 arate from its compounds that it has not come into 

 general use. 



43. Iron is a common metal with which every- 

 body is familiar. Its ores, such as hematite and 

 limonite, are abundant, and are used in immense 

 quantities for the manufacture of this useful metal. 

 It is found in all soils forming the coloring matter 

 of clays, and exists in a great variety of minerals. 

 Manganese is a metal very much like iron in its chem- 

 ical properties, but much more difficult to separate 

 from its ores.* 



44. In giving the composition of a soil it is usual 

 to state the percentage weight of the metallic oxides 

 and acid-forming oxides, as it is known that the ele- 

 ments do not exist in the soil in a free state. The 

 process of analyzing a soil, that is, of finding out its 

 constituents, is easily understood in theory, but to 

 perform the analysis requires some practice and 

 skill in chemical work. Such analyses are not so 

 useful to the farmer as was once supposed, since the 

 condition of the elements in the soil has more to 

 do with its fertility than quantity. 



45. The following analysis of a fertile soil will 

 give some idea of the relative quantity of the con- 

 stituents usually found : 



* For a more extended description of these elements the 

 student is referred to works on chemistry. 



