SEPARATION OF METALS. 151 



emetic (tartrate of antimony and potassium), (4) cliloride of 

 cadmium. Pass a few bubbles of gas from the generator through 

 each of these fluids in succession (fig. 66), and you will see 

 that in the first two glasses a black precipitate is formed, in 

 the third an orange one, and in the fourth a bright yellow 

 one. In each case the same kind of action takes place as that 

 occurring in Expt. 121, when solution of sulphuretted hydro- 

 gen was used instead of the gas itself; i.e., a metallic sulphide 

 (sulphide of copper, sulphide of lead, sulphide of antimony, 

 sulphide of cadmium) is formed, which precipitates, being in- 

 soluble. 



Expt. 159. To separate Metals from one another by means of 

 a Gas. For analytical purposes sulphuretted hydrogen is one of 

 the most convenient substances known to the chemist, in spite of its 

 repulsive odour ; the above mentioned experiments show that when 

 solutions containing copper, cadmium, antimony, and such like 

 metals are treated with sulphuretted hydrogen, the metal is so 

 chemically acted upon that it is removed from solution by being 

 converted into an insoluble compound which precipitates. Not 

 all metals, however, are thus acted upon; and in consequence 

 sulphuretted hydrogen serves as a means of separating those that 

 are acted upon from those that are not. 



Dissolve a few grains of green vitriol (sulphate of iron) in an 

 ounce of water, and add to the solution two drops of a solution of 

 sulphate of copper. The liquid will now contain two metals, 

 copper and iron. Pass a current of sulphuretted hydrogen gas 

 through the liquid for a few minutes, until the fluid smells 

 strongly of the gas after well stirring up. If too much copper 

 solution have not been added, this treatment will suffice to render 

 all the copper insoluble as a black precipitate, so that on filtering 

 the fluid and passing a little more gas through the clear filtrate, 

 no more black precipitate is formed (should this not be the case at 

 first, owing to having added too much copper, it will by and by 

 be effected by passing a sufficient quantity of gas). That the 

 black precipitate contains copper may be shown by transferring 

 the filter paper with the black precipitate to a small evaporating 

 basin, pouring on a little diluted nitric acid and warming, when 

 a solution of copper will be formed, which will give a deep blue 

 liquid on adding some strong ammonia solution to it (Expt. 1 24) ; 

 whilst a test for iron applied to the clear filtered liquid will show 

 that that metal is not removed from solution. For this pur- 

 pose a few drops of ferricyanide of potassium (red prussiate of 

 potash) may be employed, which will give a deep blue precipitate 

 (Expt. 132). 



