94 
measurement of high temperatures. 
[BULL. 54. 
ternally either with borax, or with silicate of soda or with some fusi- 
ble porcelain glazing, otherwise the vapors at once permeate the central 
tube d d and corrode the thermoelement within. The presence of 
vapor may be discovered by inserting a porcelain pipe-stem into d d 
Fig. 14a. Boilinjr-point crucible 
for pressure work. Scale, \. 
Fig. 15. Boiling-point cruci- 
ble, open tube. Scale, \. 
and rapidly withdrawing it. Metallic vapor, if present, usually coats 
the white stick with a black metallic covering, which rapidly oxidizes. 
Bi, Sb, Sn, Pb, etc., glaze the interior of the crucible in virtue of the 
fluxing power of their oxides. This corrosive action eventually becomes 
sufficiently active to eat its way through the central tube and discharge 
the contents through the bottom. 
A single crucible seldom will stand more than one ebullition, but a 
single ebullition may be prolonged many hours. If a zinc crucible be 
broken open when cold the walls are found to be covered with a coat- 
(748) 
