SEPARATION OF METALS INTO DIFFERENT GROUPS. 235 



Addition of hydrogen sulphide. This reagent is employed 

 either in the gaseous state (by passing it through the heated solution) 

 or as hydrogen sulphide water. The latter reagent answers in those 

 cases where but one metal is present ; if, however, metals of the 

 arsenic and lead groups are to be separated from metals of other 

 groups, the gas must be used. 



FIG. 26. 



FIG. 27. 



Apparatus for generating hydro- 

 gen sulphide. 



Apparatus for generating hydro- 

 gen sulphide. 



For generating hydrogen sulphide the directions given on page 107 may 

 be followed. In place of the apparatus there mentioned for generating the 

 gas, others may be used which have the advantage to the analyst that the 

 supply of gas may be better regulated. Fig. 26 shows such an apparatus for 

 the continuous preparation of the gas. It consists of three glass bulbs ; the 

 upper bulb, prolonged by a tube reaching to the bottom of the lowest one, is 

 ground air-tight into the neck of the second. Ferrous sulphide is introduced 

 into the middle bulb through the tubulure, which is then closed by a perforated 

 cork through which connection is made with the wash-bottle. Acid poured in 

 through the safety tube, runs into the bottom globe and rises to the ferrous 

 sulphide in the second bulb. Upon closing the delivery tube, the pressure of 

 the generated gas forces the liquid from the second bulb through the lower to 

 the upper, thus preventing contact of acid and ferrous sulphide until the gas is 

 used again. 



A convenient and cheaper apparatus is shown in Fig. 27. A glass tube, 

 drawn at its lower end to a small point and partly filled with pieces of ferrous 

 sulphide, is suspended through a cork (not air-tight) in a cylinder containing 

 the acid. The gas supply is regulated by closing or opening the stop-cock, and 

 also by raising or lowering the tube in the acid. 



