Polytechnic Association Proceedings. 929 



Hydrochloric acid: White, curdy precipitate, soluble in ammonia. 



Zinc^ or copper: Precipitate, white metallic silver. 



Sulphate of iron: In neutral solutions white metallic silver. 



We will now rapidly glance over the series of chemical combina- 

 tions, which we have succeeded in preparing with gold and other 

 elements by artificial means. 



Aurous oxide: Au O. Formed by precipitating protochloride of 

 gold with a cold, dilute solution of potassa. Blackish violet when 

 moist, bluish violet when dry. This oxide is permanent at ordinary 

 temperatures, but decomposes at two hundred and fifty degrees C. 



Pwple oxide of gold is formed, according to Berzelius, when a 

 powerful electric current is passed through a thin gold wire. The 

 gold burns with a green light, and is converted into vapor, which 

 deposits itself on the adjacent surfaces in the form of gold purple. 

 This purple oxide is formed even in air two hundred times rarified. 

 The presence of other bodies favors the oxidation of gold at high 

 temperatures by predisposing aflinity — the oxide formed being 

 generally red. 



When gold leaf is triturated with various substances, and exposed 

 to the blowpipe flame on a small cupel, the following results are 

 obtained: With ^oto^s^wm, the alloy oxidizes quickly as it forms, 

 and is converted into a black powder, which fuses into a violet 

 glass with silica and borax. With hydrate or nitrate of baryta, the 

 gold disappears, and colors the baryta red brown. With hydrate 

 of lime, the lime acquires a reddish or bluish gra}' color. When 

 gold leaf is placed in a crucible with hydrate of lime, in alternate 

 layei-s, and ignited, the gold disappears and the lime acquires a 

 flesh color. 



With silica containing potash, red coloring, accompanied by dis- 

 appearance of the gold. With zinc, gray powder, the gold disap- 

 pearing. With iron filings, the gold disappears quickly, and 

 yields a gray mass; and if this substance be iguited with fresh gold 

 leaf, as long as the latter disappears, a red mass is obtained, still 

 having the form of the filings, and very slightly soluble in dilute sul- 

 phuric acid, .the solution not being attended with evolution of gas 

 {T. A. Buchner Rep., 27, 1). It is in this red form that gold enters 

 into glass fluxes. 



Auric acid, or teroxide of gold: Au 0^ produced by precipitating 

 a hot aqueous solution of terchloride of gold, free from excess of 

 hydrochloric acid, with a strong excess of potash. There are many 

 other methods of preparing it. It is a brown black powder, having 



[Inst.] . 59 



