METALS. 179 



order ; oxygen, silicon, calcium, aluminium, potassium, iron, 

 hydrogen, sodium, magnesium, manganese, carbon, sulphur, 

 phosphorus and nitrogen. 



Some of these substances, with their combinations, have 

 been described in previous chapters. It is intended here, to 

 arrange them into their natural groups, and to give a general 

 description of those which have not yet been referred to. 



I. Seven of these simple bodies already described, viz. 

 oxygen, hydrogen, carbon, nitrogen, sulphur, silicon and phos- 

 phorus, are non-metallic substances. With the exception of 

 oxygen and nitrogen, they are combustible, and with the excep- 

 tion of carbon, do not exist in the rocks in their pure state. 



II. The remaining substances are Metals. These are di- 

 vided into three groups. 1. Potassium and sodium, which 

 are metallic bases of the alkalies potassa and soda. 2. Alu- 

 minium, calcium and magnesium, which are the metallic 

 bases of the alkaline earths alumina, lime and magnesia. 3. 

 Iron and manganese, which are the bases of metallic oxides. 



Potassium is a white metal, lighter than water, and so soft, 

 that it easily yields to the pressure of the fingers. It is the 

 most combustible of the simple substances. This is owing 

 to its affinity for oxygen, which it will abstract from water 

 with such rapidity, as to burn upon its surface with a beauti- 

 ful purple flame. It thus decomposes the water, and forms 

 the alkali potassa, which is the basis of potash. It is widely 

 disseminated in the rocks, though not in very large quanti- 

 ties. 



Sodium is also a white metal, like silver, lighter also than 

 water, but a little heavier than potassium, to which it is simi- 

 lar in texture and consistency. It is less combustible than 

 potassium, but will also rapidly decompose water to obtain 

 its oxygen, with which it forms the pure alkali soda, the ba- 

 sis of all the compounds of soda, known under the names of 

 common salt, glauber's salts, etc. 



Magnesium is a pure white metal resembling silver, very 



