reduction-tube

from The Century Dictionary.

• noun A small tube of difficultly fusible glass, closed and expanded to a bulb at one end, in which, by heating with suitable reagents, such substances as mercury and arsenic may be reduced to the metallic state, volatilized, and condensed upon the surface of the colder part of the tube. The name is also applied to a hard glass tube open at both ends and having one or more bulbs blown in the middle of the length, in which oxid of copper, of iron, or other metal may be reduced to the metallic state by heating in a current of hydrogen carried through the tube.