PLATINUM. 61 



SILVER. 



Occurs native, and in combination with gold and other metals ; 

 also with chlorine- and sulphur. One of the most abundant silver 

 ores is the argentiferous galena. It is extracted either by amalga- 

 mation, or by cupellation, — the latter process being only applicable to 

 the combinations with lead. Pure silver may be procured by preci- 

 pitation from a solution in nitric acid, by means of chloride of sodium ; 

 and heating the resulting chloride with carbonate of soda. 



Prop. — The whitest of all metals ; has a brilliant lustre ; is very 

 malleable and ductile ; the best conductor of heat and electricity ; 

 sp. gr. 10-5 ; is not acted upon by moisture or air, unless sulphur 

 be present; its proper solvent is nitric acid; Symb. Ag.; forms two 

 or three oxides, of which the protoxide acts as a base. The most 

 important salt is the 7iitrate, or lunar caustic^ — made by dissolving 

 silver in nitric acid, and evaporating. The crystals are colourless ; 

 very soluble in water ; the solution becomes dark coloured when 

 exposed to light, probably from the formation of an oxide. 



The arbor Diana is made by suspending mercury in a solution of 

 the nitrate : the silver is precipitated in the form of crystals. It is 

 also precipitated by the chlorides, phosphates, chromates, arseniates, 

 arsenites, and several of the metals. 



The best test is chlorine, or a soluble chloride. 



Silver forms compounds with chlorine, iodine, and sulphur. 



PLATINUM. 



Occurs only in the metallic state, generally combined with other 

 metals, particularly palladium, osmium, rhodium, and iridium. It 

 is found in grains, which are converted into masses by intense heat 

 and pressure. 



Projp. — It has a whitish colour ; very malleable ; the most ductile 

 of metals ; the heaviest body in nature ; sp. gr. 21*5 ; is soft ; can 

 be welded, like iron ; undergoes no change by exposure to air, mois- 

 ture, or the highest heat of a smith's forge ; can only be fused by 

 galvanism or the oxy-hydrogen blowpipe ; it is dissolved by nitro- 

 muriatic acid. In its habitudes with oxygen, chlorine, and sulphur, 

 it resembles gold. 



Spongy platinum has the singular property of causing the union 

 of oxygen and hydrogen gases, producing combustion. 



Tests. — A solution of chloride of potassium produces with chlo- 

 ride of platinum a double chloride, of a pale yellow colour ; sal 

 ammoniac gives with it a yellow precipitate, which, by heat, yields 

 spongy platinum. 



The metal iridium is said to be a little heavier than even platinum; 

 its sp. gr. being 21 '8, The other metals associated with platinum 

 (palladium, osmium, and rhodium), need no particular description. 



Q 



