160 PRACTICAL LESSONS IN SCIENCE. 



ture, but it occurs most abundantly as the silver sulphide, com- 

 bined with lead sulphide and other substances. It also- occurs as 

 the chloride and bromide. The process of obtaining silver from 

 its ores is somewhat complicated and costly. In a general way 

 it is accomplished by heat aided by chlorine from common salt, 

 and by the dissolving power of mercury or lead, or both. 



Silver is used for coin, for plate and for ornaments. Pure 

 silver is so soft that it is usually hardened by the addition of 

 from 7 per cent, to 10 per cent, of copper, so that coins and sil- 

 verware are in fact alloys of silver and copper. It is exten- 

 sively used in plating ware made of cheaper metals or alloys. 

 The chloride of silver, formula, AgCl, is one of the richest ores 

 of silver, and is the form into which silver is usually converted 

 in extracting it from ores. The chloride, bromide and oxide of 

 silver are insoluble in water and change color on exposure to 

 the light. There are many other compounds of silver of which 

 silver nitrate, AgN0 3 , called lunar caustic, is the most important. 

 It is used in surgery, and as it blackens on exposure to light is 

 made the basis of indellible inks. Silver may be precipitated 

 from solutions by zinc, copper, mercury and other metals. 



The chemical changes which silver salts undergo on exposure to 

 light is the basis of the art of photography. To get a photograph 

 on glass pour over the glass a mixture of collodion and potas- 

 sium iodide, which forms a thin film over the glass, which is 

 then placed in a bath of silver nitrate, here a layer of silver 

 iodide is formed in the film. The plate is then exposed in the 

 camera to the action of light from some object to be photo- 

 graphed. When removed from the camera no image is percepti- 

 ble, but on treating the film with a "developer" as a solution 

 of gallic acid in alcohol and acetic acid, the illuminated por- 

 tions appear black while the shaded portions retain the yellow- 

 ish color of the iodide. When the details come out clearly, the 

 developer is washed off and the film treated with a solution of 

 sodium hyposulphite, which dissolves out the iodide that has not 

 been affected by the light, then the hyposulphite is washed away 

 and the plate dried and varnished to protect the film. 



