176 



Allen, Crenshaw, Johnston, and Larsen- 



details also. The sulphide has to be very carefully purified. 

 Obviously, auy other product which could reduce the solution, 

 or anything like ferrous sulphate or ferric hydroxide which 

 could in any way increase the concentration of the ferrous iron, 

 would interfere with correct conclusions. In the main, Stokes' 

 procedure has been followed, though a new apparatus for 



Fig. 3. 



-f-i 



Fig. 3. Apparatus for washing substances out of contact with. air. 



washing substances out of contact with the air has been 

 devised and used. This apparatus, which is shown in the 

 adjoining figure (fig. 3), has proved of very general utility and 

 convenience. A consists of two parts, the lower of brass, the 

 upper of glass cemented into a brass . ring. F is a ground 

 joint between this ring and another similar ring soldered to 



