FARADAY'S RUBY GOLD. 133 



formed by the action of the mercury which becomes dissolved as 

 the silver is removed from solution. If, after all the silver has thus 

 been precipitated, the liquid (now containing solution of nitrate of 

 mercury) be allowed to stand with strips of clean copper immersed, 

 the mercury will be again thrown out of solution, forming a white 

 film on the surface of the red copper, from which, if thick enough, 

 small drops of fluid mercury may be wiped off by pressure between 

 the fingers; by and by all the mercury is thus reprecipitated, 

 whilst copper dissolves during the action. Finally, if the result- 

 ing solution of "nitrate of copper" be treated with metallic iron, 

 the copper is in turn precipitated again, whilst the iron dissolves, 

 as in Expt. 132. 



If a bundle of thin copper wires be placed in a solution of silver 

 nitrate, metallic silver will be deposited in crystals all over the 

 surface of the copper, forming a sort of silver tree resembling the 

 "arbor Saturni" of the last experiment, some of the copper 

 becoming dissolved during the process. 



In order to precipitate a metal from a solution of one of its 

 compounds, it is not always necessary to use another metal as in 

 the last experiments ; thus, for example, gold may be precipitated 

 by means of fluids. 



Expt. 135. To precipitate Metallic Gold from Solution by 

 means of a Fluid. Obtain a small quantity of solution of chloride 

 of gold (a rather costly substance), and add it to a much larger 

 bulk of water. Prepare a solution of sulphurous acid (Expt. 117), 

 or one of sulphate of iron (sometimes called "green vitriol") and, 

 add a little of it to the diluted solution of gold; a very finely 

 divided precipitate will be formed, which at first will remain sus- 

 pended in the water, giving it a peculiar greenish hue when 

 looked through, but will by and by subside to the bottom of the 

 vessel as a brownish-yellow powder ; this powder is pure gold in a 

 "spongy" condition; if collected by pouring off the fluid care- 

 fully, stirring up with water, allowing to subside so as to wash 

 away the adhering solution, and finally drying with a gentle heat, 

 it looks more like iron rust than anything more valuable ; but if 

 "burnished" or pressed with a hard smooth surface it becomes 

 bright and yellow, and like ordinary gold in appearance. Many 

 other metals besides gold may be precipitated from solution in 

 similar fashion, by using appropriate chemicals ; glass objects are 

 often coated with silver, so as to produce mirrors by processes of 

 this kind (Expt. 198). 



Expt. 136. To obtain precipitated Gold apparently in the form 

 of a blood-red Fluid. Make an extremely weak solution of chloride 

 of gold, and put it in a white glass bottle. Get a small fragment 



