EARTHS. 285 



(h.) Alumina and baryta, in equal parts, boiled together in water, 

 'are dissolved. 



(i.) Five parts of strontia being boiled on 1 of alumina, a portion is 

 dissolved, and a compound of strontia and alumina is left undissolved ; 

 they unite also by fusion. 



(j.) Alkaline solution of alumina, added to lim_e water, produces 

 an insoluble precipitate of lime and alumina. 



(k.) The same alkaline liquor boiled on lime dissolves no more 

 than the water alone will dissolve ; if alumina be mixed with the lime, 

 much more lime is taken up than before. 



(I.) Mixture of alumina and lime, the latter being in excess, melts 

 under oxygen gas, directed upon burning charcoal, but in no propor- 

 tions in a common furnace. 



(m.) Alkaline solution of silica, with alkaline solution of alumina, 

 on being mixed, gives a precipitate of silica and alumina, which fuses 

 with an intense heat, into a milky glass or enamel. 



(n.) Not soluble in alkaline carbonates. 



(o.) Alumina unites by fusion with the fixed alkalies, and with 

 most of the earths. Henry. 



(p.) This earth attracts coloring matter powerfully see dyeing, 

 under vegetables. 



(q.) CONTRACTS PERMANENTLY IN THE FIRE ; and becomes so 

 hard as to give fire with steel. This is from an intimate union of the 

 molecules, and especially when silica is present, as in the natural 

 clays. 



Wedgewood's pyrometer depends on this fact. See heat, and 

 the means of measuring heat. 



4. POLARITY. Electro positive ; it is separated at the negative 

 pole in the galvanic circuit. 



5. COMBINING WEIGHT. 18 according to Dr. Thomson and 

 Gay-Lussac, but chemists are not perfectly agreed as to this number. 



6. DISTINCTIVE CHARACTERS. 



(a.) Plastic with water, and imparts this property to large mix- 

 tures of other earths. 



(b.) Precipitated as a hydrate, by alkaline carbonates, and by 

 pure ammonia. 



(c.) Precipitated by pure potassa and soda, and immediately re- 

 dissolved by an excess of those alkalies ; some choose to call this an 

 acid property, but the alkali is not fully neutralized. 



6. NATURAL HISTORY AND USES. 



In various forms and combinations, it is one of the most abundant 

 substances in nature. Clays are composed of alumina, for their 

 characteristic ingredient, mixed with silica, oxide of iron and other 

 substances. Those clays which, when burned, become red, contain 

 oxide of iron, as is seen in our red bricks. 



