SILVER, GOLD, PLATINUM 521 



in excess to silver nitrate solution. This solution gives a compact 

 deposit on electrolysis. The anode is of silver, so that the silver 

 in the solution is replenished as quickly as it is deposited. 



Mirrors are silvered by cleaning the surface and pouring over 

 it a solution containing silver nitrate, ammonium hydroxide, and 

 a reducing agent like formaldehyde CH 2 O, or grape sugar: 



4AgOH + CH 2 -> 3H 2 + 4Ag J, + C0 2 . 



The film of silver adheres to the glass and is washed, dried, and 

 varnished. 



Silver Nitrate AgNO 3 . This exceedingly soluble salt is 

 deposited from hot solution (p. 520) in colorless crystals. Its 

 solution is neutral in reaction, which shows that silver hydroxide 

 is not a weak base. It melts easily and is cast in sticks (lunar 

 caustic), which are used in cauterizing sores. It is the chief source 

 of the other compounds of siher. It is used in some hair dyes, 

 and in indelible ink. In the latter case the organic matter in 

 the goods reduces it, with the help of light, to metallic silver. 



Silver Halides. Silver chloride AgCl is precipitated (white) 

 when a soluble chloride is added to a solution of a salt of silver: 



AgN0 3 + KC1 - AgCl I + KN0 3 . 



Silver bromide AgBr is precipitated with bromide-ion and silver 

 iodide Agl with iodide-ion. These compounds have a yellowish 

 tinge. The chloride and bromide are easily dissolved by am- 

 monium hydroxide solution, giving thejgomplex ion Ag(NH^ 2 + 

 (see p. 513), and also by sodium thiosulphate solution Na^Os 

 (" hypo ). 



All the halides of silver are decomposed by light, which liber- 

 ates the halogen, and finally leaves metallic silver. 



Photography. The taking of a photograph involves four 

 processes preparation of the plate, exposure, development, and 

 fixing. 



