garths. 171 



into vapour. The second characteristic is indis- 

 pensable to an earthy substance, and in a great 

 measure peculiar to this class. The bodies which 

 come under the class of earths are nine in num- 

 ber, viz. 



1. Lime. 6. Strontian. 



2. Alumina (or clay). 7. Yttria. 



3. Silica, or Flint. 8. Glucina. 



4. Magnesia. 9. Zirconia. 



5. Barytes. 



Lime, Barytes, and Strontian have been shown 

 by Sir Humphry Davy to have metallic bases, and 

 most of the other earths have been analogically 

 considered to be the oxides of peculiar metals. 



Some of these earths are much more plentiful 

 than others, and the great mass of this globe 

 may be said to be composed of lime, flint, and 

 clay, variously mixed and disposed in the dif- 

 ferent species of stony bodies. 



The surface of the earth evidently consists of 

 a confused mixture of decayed animal and ve- 

 getable substances, and earths rudely united to- 

 gether; but when we have got below the sur- 

 face, we find the materials of the globe arranged 

 in a regular manner. Sometimes, indeed, we 

 find heaps of stone, which do not consist of 

 layers, but are confused masses of unequal thick- 

 ness ; these are called rocks. The strata are in 

 general extended through a whole country, and 

 perhaps through the globe itself. Thus, in dig- 

 ging for coal, the workmen first come to a bed of 

 clay, next to a bed of sand, for the thickness of 



