SODIUM 7 



In the form of carbonate and sulphate it occurs as an incrusta- 

 tion on the surface of the ground, or disseminated throughout 

 the soils in poorly drained portions of arid countries, giving 

 rise to the so-called '^alkali soils," for which such regions are 

 frequently noted. As a nitrate, sodium occurs in the desert 

 regions of Chili, forming the soda nitre so valuable for fer- 

 tilizing purposes. 



Manganese is, next to iron, the most abundant of the heavy 

 metals. It occurs as an oxide, carbonate, or in combination with 

 two or more other elements as a silicate. 



Barium is found mainly combined with sulphuric acid, to 

 form the mineral barite or heavy spare. It sometimes occurs 

 as a carbonate, and more rarely as a silicate. 



Phosphorus, although existing in comparatively insignificant 

 proportions, is nevertheless an important element. In nature 

 it occurs only in combination with various bases, principally 

 lime, to form phosphates. In this form it is found in the bones 

 of animals, the seeds of plants, and constitutes the essential 

 portions of the minerals apatite and phosphorite. Though 

 small in proportion, phosphorus is a very important constituent 

 of any fertile soils. Its chief source, in the older, crystalline 

 rocks, is the mineral apatite. Where found in the secondary 

 rocks, as limestones and marls, it is evidently derived from 

 animal remains. (See p. 131.) Analyses have shown that the 

 amount of phosphorus in rocks rarely exceeds 1% (calculated 

 as P2O5), and usually falls much lower, being most abundant 

 in the basic eruptives. The following table will serve to show the 

 small percentages of this constituent in rocks of various kinds: 



Granite 0.07 to 0.25% 



Diorite 0.18 to 1.06% 



Basalt 0.03 to 1.18% 



Limestane 0.06 to 10.00% 



Shale 0.02 to 0.25% 



Sandstone 0.00 to 0.1 % 



, Carbon. — Of the solid elements occurring free, or uncom- 

 bined, carbon is by far the more abundant, being found in the 

 forms known as diamond and graphite, or when quite impure 

 as coal. In combination as a dioxide (CO2), it forms the well- 

 known carbonic acid gas, which, like oxygen, is a powerful 

 agent in bringing about important changes in the rocks with 



