ALUM. 



into a white powder oxide of magnesium, or magnesia. Combustion is still 

 more active in oxygen, and powder of magnesium placed 

 in a jar filled with this gas produces a perfect shower of 

 fire of very beautiful effect. To give an idea of the 

 lighting power of magnesium, we may add that. a wire of 

 this metal, which is -f^ of a millimetre in diameter, 

 produces by combustion a light equal to that of seventy- 

 four candles. 



The humble earth of the fields the clay which 

 is used in our potteries, also contains aluminium, that 

 brilliant metal which is as malleable as silver, and unspoil- 

 able as gold. When clay is submitted to the influence 

 of sulphuric acid and chloride of potassium, we obtain alum, which is a 

 sulphate of alumina and potash. Alum is a colourless salt, which crystallizes 

 on the surface of water in beautiful octahedrons of striking regularity. Fig- 



Fig. 309. Calcined alum. 



Fig. 310. Preparation of metallic iron. 



308 represents a group of alum crystals. This salt is much used in the 

 colouring of fabrics ; it is also used for the sizing of papers, and the clarifica- 

 tion of tallow. Doctors also use it as an astringent and caustic substance. 

 When alum is submitted to the action of heat in an earthenware crucible, it 

 loses the water of crystallization which it contains, and expands in a singular 

 manner, overflowing from the jar in which it is calcined (fig. 309). 



Iron, the most important of common metals, rapidly unites with oxygen, 

 and, as we know, when a piece of this metal is exposed to the influence of 

 damp air, it becomes covered with a reddish substance. In the well-known 

 experiment of the formation of rust, the iron gradually oxidises without its 

 temperature rising, but this combination of iron with oxygen is effected much 

 more rapidly under the influence of heat. If, for example, we redden at the 

 fire a nail attached to a wire, and give it a movement of rotation as of a 

 sling, we see flashing out from the metal a thousand bright sparks due to the 

 combination of iron with oxygen, and the formation of an oxide. Particles 

 of iron burn spontaneously in contact with air, and this property for many 



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