122 SCIENTIFIC AMUSEMENTS. 



Gold is a metal unlike most others, being entirely unaffected 

 by nitric acid (vide Expt. 230). 



Expt. 120. To separate Silver from Copper. Dissolve a 

 silver coin in nitric acid, as in the last experiment, and add to the 

 solution some clear brine (solution of common salt or chloride of 

 sodium). This substance will react by double decomposition on 

 the nitrate of silver, as in Expt. 37, forming chloride of silver, 

 which, being insoluble, will precipitate in curdy flakes. When 

 these have subsided, the clear liquid may be poured off, and treated 

 with iron by immersing a knife blade in the fluid, when the copper 

 will be thrown down as a red film on the iron (Expt. 9). To obtain 

 the silver, stir the chloride of silver up with water, let it subside, 

 and pour away the liquid, and repeat the operation two or three 

 times to remove all the copper solution ; this is called " washing 

 by decantation." Now pour some solution of caustic soda on the 

 chloride of silver, and add a good-sized lump of sugar, or better, a 

 teaspoonful of milk sugar (the sugar contained in milk). Keep 

 the whole hot or boiling for some hours, and then wash the 

 blackish powder resulting by decantation again ; the chloride of 

 silver will have become mostly transformed into spongy metallic 

 silver by the treatment, so that after washing, the powder may be 

 treated with nitric acid, which will dissolve almost the whole. 

 The solution should be filtered to separate any chloride of silver 

 that has not become decomposed during the treatment with soda 

 and sugar, and may then be evaporated to dryness, furnishing pure 

 silver nitrate. 



Expt. 121. Separation of Solid Substances from Solutions 

 (accompanied by Chemical Changes) as Precipitates of various 

 Colours. In the preceding experiments we have had instances of 

 chemical action taking place between substances insoluble in water 

 (oxide of magnesium, or metallic copper, &c.) and other substances 

 soluble in water (hydrochloric acid or nitric acid), with the result 

 of producing products of chemical change soluble in water. This 

 kind of action may be inverted, so that by taking two substances, 

 both soluble in water, and bringing them together, one or more 

 different substances insoluble in water may be produced by 

 chemical action. In such cases the insoluble product is usually 

 said to be precipitated ; some illustrations of this kind of action 

 have already come before us, e.g., in Expt. 37, where a white pre- 

 cipitate was produced by the chemical action of nitrate of silver 

 solution on brine ; and in Expt. 57, where a blue precipitate of 

 prussian blue was formed by the chemical action of ferrocyanide 

 of potassium solution on solution of perchloride of iron. In the 

 first of these cases a white precipitate is formed on mixing two 



