60 



CHEMISTRY. 



carbonate of potassa to a solution of alum ; the hydrate of alumina 

 is precipitated. 



Properties. — Has no taste or smell ; feels pasty to the tongue ; is 

 very insoluble and infusible ; has a strong affinity for water ; acts 

 but feebly as a base. 



The most important salt of alumina is alum — a double sulphate 

 of alumina and potassa : either soda or ammonia may be substituted 

 for the potassa in alum. The salts of alumina are useful as mor- 

 dant& in dyeing. Alumina forms the basis of porcelain and earthen- 

 ware. Tests for alumina.— Csiustic potash and soda give with 

 it white gelatinous precipitates ; — the well-known characteristics of 

 alum ; — and the blue colour given by nitrate of cobalt by heat. 



The remaining metals of the earths are of no particular im- 

 portance. 



SECTION IV. 



CLASS IV. 

 METALS PROPER. 



This class may conveniently be subdivided into three orders, viz. 



1 . Metals whose oxides are reduced by heat alone, — noble metals ; 



2. Metals whose oxides form powerful bases; 3. Metals whose 

 oxides form weak bases or acids. 



ORDER I. METALS WHOSE OXIDES ARE REDUCED BY HEAT. 



GOLD. 



Occurs either pure, or combined with silver or copper ; is usually 

 separated from impurities by amalgamation with mercury, which is 

 afterwards driven off by heat. May be obtained pure by dissolving 

 in nitro-muriatic acid, and then precipitating by a protosalt of iron. 



Prop. — It has a well-known yellow colour ; is the most malleable, 

 and one of the most ductile of all metals ; is not tarnished by either 

 air or moisture; sp. gr, 19-3. Symb. (Aurum,) Au. The only 

 solvent for gold is a mixture of nitric and hydrochloric acids, or 

 rather, the solution of chlorine which thence results. An etherial 

 solution of gold is made by agitating ether with the strong aqueous 

 solution ; this may be used for gilding. If. protochloride of tin be 

 added to the solution of gold, the purple powder of Cassius is thrown 

 down. Gold forms compounds with oxygen, chlorine, iodine, and 

 sulphur. Gold coins are always alloyed with copper or silver, which 

 increase its hardness. The best test for gold is the protochloride of 

 tin. 



