C 137 



parts obtained are compound s; and they act as com- 

 pounds in nature: it is in this state, therefore, that I 

 shall describe their characteristic properties. 



1. Silica, or the earth of flints, in its pure and 

 crystallized form, is the substance known by the name 

 of rock crystal, or Cornish diamond. As it is procur- 

 ed by chemists, it appears in the form of a white 

 impalpable powder. , It is not soluble in the common 

 acids, but dissolves by heat in fixed alkaline lixivia* 

 It is an incombustible substance, for it is saturated 

 with oxygene. I have proved it to be a compound of 

 oxygene, and the peculiar combustible body which I 

 have named silicum; and from the experiments of Ber- 

 zelius, it is probable that it contains nearly equal 

 weights of these two elements. 



2. The sensible properties of lime are well 

 known. It exists in soils usually united to carbonic 

 acid; which is easily disengaged from it by the attrac- 

 tion of the common acids. It is sometimes found 

 combined with the phosphoric and sulphuric acids. 

 Its chemical properties and agencies in its pure state 

 will be described in the Lecture on manures obtained 

 from the mineral kingdom. It is soluble in nitric and 

 muriatic acids, and forms a substance with sulphuric 

 acid, difficult of solution, called gypsum. It is not 

 soluble in alkaline solutions. It consists of one pro- 

 portion 40 of the peculiar metallic substance, which I 

 have named calcium; and one proportion 15 of oxy* 

 gene. 



3. Alumina exists in a pure and crystallized state 

 In the white sapphire, and united to a little oxide of 



