138 METALS AND THEIR COMBINATIONS. 



226. What is plaster of Paris, and what is gypsum; what are they 

 used for? 



227. State composition and mode of manufacturing bleachiug-powder; 

 what are its properties, and how do acids act upon it? 



228. What is bone-black, bone-ash, acid phosphate, and precipitated 

 tricalcium phosphate? How are they made? 



229. Give tests for barium, calcium, and strontium; how can they be 

 distinguished from each other? 



230. Which compounds of barium and strontium are of interest, and 

 what are they used for? 



24. ALUMINIUM. 



Al = 27.4. 



Aluminium is the representative of the metals of the earths 

 proper; all other members of this class are found in nature in 

 very small quantities, and are of scientific interest only. 



Occurrence in nature. Aluminium is found almost exclusively 

 in the solid mineral portion of the earth ; .rarely more than 

 traces of aluminium compounds are found dissolved in water, 

 and the occurrence of aluminium in either the vegetable or 

 animal organism seems to he purely accidental. 



By far the largest quantity of aluminium is found in combi- 

 nation with silicic acid in the various silicated rocks forming 

 the greater mass of our earth, such as feldspar, slate, basalt, 

 granite, mica, hornblende, etc., or in the various modifications 

 of clay formed by their decomposition. 



The minerals known as corundum, ruby, sapphire, and emery are 

 aluminium oxide in a crystallized state and more or less colored 

 by traces of other substances. 



Metallic aluminium is obtained by the decomposition of alu- 

 minium chloride by metallic sodium : 



A]C1 6Na = 6NaCl 2A1. 



Aluminium is an almost silver-white metal of a very low 

 specific gravity (2.6); it is capable of assuming a high polish. 

 and for this reason is used for ornamental articles. 



Some of the alloys of aluminium are now used in the arts, 



