barus] CALIBRATION OP ELECTRICAL PYROMETERS. 93 
naces may be placed on the same bed-plate, in a row. Each furnace is 
provided with its own burner, all of which are fed from the same bellows 
and the same gas-supply (see frontispiece under D). It is best for this 
purpose to attach the bellows (Fletcher's pattern) to an engine, on a 
very short crank. The pressure of air may then be regulated by in- 
creasing the length of the crank. Burners constructed on the plan 
described at length below (page 183), only on a smaller scale, are prefer- 
able. They do not ex|)lode back. For very high temperatures two 
and even three such burners may be made to impinge, on the same 
crucible. For cadmium or zinc a single burner is more than sufficient. 
At high temperatures the efflux hole D may be partially closed with 
asbestos. The products of combustion escape uniformly on all sides 
around the plaue where the furnace-body and furnace-lid meet. A ring 
of asbestos, placed around the crucible to protect it .from the flame of 
the burner, is soon fluxed down upon it, and is apt tc destroy the cru- 
cible. A ring of baked fire-clay, however, is good. 
The crucible shown in Fig. 14a is intended for work in which the 
variations of boiling point and pressure are to be investigated. It is 
made of refractory porcelain and glazed within. The lid cab fits pretty 
snugly into the crucible efd, so that the two may be sealed hermetically 
at the joint c d by sodium tungstate (Gooch) or other material. The 
tube at a is in connection with the air-jminp. Such cr.icibles are avail- 
able for the ebullition of salts of selenium, cadmium, zinc, and probably 
antimony and bismuth in vacuum. Being made of porcelain they can 
be more elegantly shaped than fire-clay crucibles, but they become se- 
riously viscous at a lower temperature. 
A second form of boiling crucible is shown in Fig. 15. It differs from 
Fig. 14 only in this respect, that the central tube d d, which in Fig. 14 
is closed just below the surface of the boiling metal, in Fig. 15 extends 
quite through the crucible and out of the top. The latter form has the 
advantage that the degree of constancy of temperature along the length 
of the tube may be explored by inserting an insulated thermo-element. 
The part of this tube above the surface of ebullition is closed during the 
measurement with a fire clay plug, and at the top with asbestos wick- 
iug. The form shown in Fig. 15 may also be used for annealing wires 
at definite high temperatures, by drawing them through the zone of 
ebullition by clock-work (cf. Fig. 10a). This form has therefore many 
decided advantages over that in Fig. 14, with the one serious disadvan- 
tage of being much more fragile. Fortunately it appears from the data 
below, that the more practical form, Fig. 14, is quite reliable as regards 
accurate value of the boiling points attained. 
In all cases the substance to be boiled, 1c k, must surround the tubes 
d d below the plane of the burners, even more than has been shown in 
Fig. 14. When this is the case no part of the tube d d will be at a tem- 
perature higher than the boiling point of k k, a desideratum. 
In case of Zn, Cd, and other metals the crucible must be glazed in- 
(747) 
