SOME RECENT TRIUMPHS 



rises to the surface when stirred into a crucible contain- 

 ing the hot alumina solution : moreover, it rises to the 

 surface and remains there as a shield to protect the 

 workmen against the heat of the solution. It serves 

 yet another purpose, as the powdered alumina may be 

 sifted upon it and left there to dry before being stirred 

 into the crucible. A most ingenious yet simple device 

 tells the workman when any particular crucible is in 

 need of replenishing. A small, ordinary, incandescent 

 electric-light bulb is placed in circuit between the poles 

 that convey the electric current through the alumina 

 solution. So long as the crucible contains alumina, 

 the bulb does not glow, because twenty volts of elec- 

 tricity are required to make it incandescent, whereas 

 seven volts pass through the solution. But so soon as 

 the alumina becomes exhausted, resistance to the 

 current rises in the cryolite solution and, as it were, 

 dams back the electric current until it overflows into 

 the wire at sufficient pressure to start the signal lamp. 

 Then it is necessary merely for a workman to stir 

 into the solution the dry alumina resting on the sur- 

 face, along with the coke that supports it. This, of 

 course, reestablishes the electrolytic process; the lamp 

 goes out and the coke, unaffected by its bath, rises to 

 the surface to support a fresh supply of alumina. 



Such a process as this, contrasted with the usual 

 methods of smelting metals in fiercely heated furnaces, 

 seems altogether wonderful. Here a pure metal is ex- 

 tracted from the clayey earth of which it formed a part, 

 without being melted or subjected to any of the familiar 

 processes of the picturesque, but costly, laborious, and 



