BAitus.l CALIBRATION OF ELECTRICAL PYROMETERS. 85 
the body of plaster within and a layer of plaster braced against the 
iron cap without. The eap cc carries an iron tube, dd, closed above 
but open below, and occupying- ;l central or axial position with respect 
to the glass tube, into which it projects about two inches (dear. The 
top of the lamp-chimney is closed by a suitable cork, ee, doubly perfo- 
rated, through the central hole of which is inserted a wide glass tube, 
hi, partially closed above by a loosely fitting- stopper. Through the 
other perforation passes a glass tube, fg, by aid of which some gas (N 2 , 
C0 2 ) may be introduced. The glass aaaa is partially filled with the 
substance whose boiling point is to be used (in the present instance 
mercury), only enough being poured in to submerge the central tube dd 
with the exception of about 0.5 cm of its head. To keep the metal in 
ebullition, use is made of Dr. Wolcott Gibbs's ring-burner, 1 rr, the flame 
of which is properly regulated. Very thin copper or brass gauze, or 
copper-foil, m m, surrounding the part of the glass tube encircled by the 
ring-burner, is sufficient to almost completely obviate the dangers of 
breakage; and a circular screen of thick asbestos, nn, bent in the shape 
of an inverted cone protects the top of the tube dd from direct radia- 
tion. Above nn it is well to surround the tube aaaa with a thick jacket 
of asbestos, pppp, extending as far down as may be without shutting 
the surface of boiling mercury entirely out from view. The mercury 
which condenses on the sides of the tube falls back in small drops into 
the mass JcJc below. The process is therefore continuous. 
The properly insulated thermo-couple is introduced into dd from be- 
low, and the hot junction is pushed forward quite as far as the top'of 
the tube d d and slightly above the surrounding surface of mercury. A 
screen may be fastened a little below the cap c c to shut off all radiation 
from the cold junction, which is submerged in oil. 
The apparatus for sulphur in Fig. 8 differs from that in Fig. 7 only 
in that the wide central tube hhii has within it a second glass tube, 
qt, partially closed above with a cork. This second tube whenever the 
passage below is stopped up by the distillation of sulphur may be at 
once removed and a similar clear tube inserted. A slow current of dry car- 
bonic acid gas entering at g passes through the apparatus during ebul- 
lition. Subsequent experiments showed that with suitable changes in 
the apparatus the tube hhii as well as gas current could be dispensed 
with. This will be referred to again below. The sulphur condenses on 
the sides of the tubes and by far the greater part runs back into the 
mass hlc below. There is a line of deraarkation encircling the tube 
where the temperature is the melting point of sulphur. 
For liquids with a boiling point below that of mercury and sufficiently 
low not to char a cork, the boiling tube may be considerably simplified 
in the way shown in Fig. 9. Here both ends of the lamp-chimney are 
closed with a cork centrally perforated to admit a long glass tube, d d, 
1 The Gibbs ring-burners were introduced into this laboratory by Drs. Goocb and 
Cbatard, and have since become invaluable. 
(739) 
