260 EARTHS. 



these four earths constitute the great mass of our mountains, rocks, 

 stones, gravel, and soil, and yere the five others annihilated, it would 

 not sensibly diminish the volume of the crust of the globe. Baryta 

 and strontia exist, however, in some quantity, and baryta, especially 

 combined with sulphuric acid, is of frequent occurrence, although it 

 is generally confined to veins in the rocks. 



As chemical reagents, lime and baryta are of signal utility ; stron- 

 tia possesses similar properties, but has, in comparison with those 

 earths, little that is peculiar, or that gives it a ground of preference. 

 Silica, alumina, and magnesia are of limited use in scientific chem- 

 istry, but they are of vast importance in the arts, and along with 

 lime, are the foundation of the vegetable kingdom, and of agricul- 

 ture ; as our best soils consist of different proportions of these earths ; 

 and the varying qualities of soils, although modified in an important 

 degree by moisture and by animal and vegetable matter, and other 

 causes, are characterized chiefly by the predominant earths. 



The preceding sketch has been presented, that the student might 

 not fail to obtain a just idea of the important natural order of earths, 

 which it is difficult to define by unexceptionable chemical characters ; 

 but there is no difficulty in giving clear discriminations, provided we 

 divide the earths into groups.* 



The divisions under which the earths will be described, are 



1. Alkaline earths. 



2. One earth of a sub-alkaline character. 



3. Earths proper. 



ALKALINE EARTHS. 



LIME BARYTES STRONTIA. THEIR GENERAL CHARACTERS. 



(a.) Soluble in water, but much less so than the alkalies. 



(b.) Acrid and caustic ; in light powder, irritate the nostrils, and 

 produce sneezing. 



(c.) Test colors affected by them, as by the alkalies. 



(d.) Differ from the alkalies in their very difficult fusibility, but fu- 

 sible by the compound blowpipe, and by galvanism. 



* Perhaps the only characters that will strictly apply to them all, are these 

 1. They are, when prepared pure by art, white powders. 2. They are not volatile 

 by heat, and are remarkably difficult to melt, and are, both when pure, and when in 

 combination with each other, in the stones and rocks, the most infusible and unalter- 

 able bodies that are generally known to mankind. 3. They have oxygen for a com- 

 mon principle, united, in each earth, to a peculiar metallic or combustible base. It 

 is true (as suggested by a friend,) that some of the proper metallic oxides, would be 

 covered by these characters, e. g. the oxides of columbium, titanium and cerium; 

 but still, most of our artificial divisions, fail of rigorous exactness; the oxides them- 

 selves graduate into the acids, but no one for that reason thinks, of blending them. 

 There can be no good objection to dividing the numerous class of oxides into con- 

 venient orders, which are also in a great measure natural. See Introduction, p. 3. 



