barus] CALIBRATION OF ELECTRICAL PYROMETERS. 95 
iug of metal consisting of solidified drops of zinc, which were distilled, 
recondensed, and ran back again down the sides. In the same way the 
central tube is kept covered with a coating of zinc. Zinc dust escapes 
at the joint between lid and crucible, and eventually melts, forming an 
impervious joint. Zinc dust also escapes where the hydrogen tube 
enters the lid. In case of zinc and cadmium, this is an excellent cri- 
terion of ebullition. In all cases the burners are to be shut off until the 
escape of metallic dust at h is only just apparent. Metals like Sb, Bi, 
have no such boiling point criteria, and whether or not the metal has 
boiled becomes a matter of conjecture. Tin charges slag so heavily that 
the metal is soon jacketed with a thick viscous coat and the state of 
the metal under it can not be known. 1 
INSULATORS. 
A very essential part of the thermo-element is the insulator. The 
device which after very many trials I finally adopted therefore de- 
serves careful description here. These iusulators are thin stems (0.45 crn 
in diameter, or larger), contaiuing two parallel canals, as far apart 
(0.20 cm ) as possible, and about 0.1 cm in diameter each. In order that 
these stems may be of value, they must be made in a way which affords 
a perfect guaranty that throughout the length of the insulator the 
canals nowhere coalesce. The following machine, Figs. 1G, 17, by aid 
of which insulators of almost any diameter and with any number of 
holes or canals may be made in lengths of 25 cm to 30 cm or more, gives 
full warrant to this assumption. The tubes are simply pressed after 
the well-known manner used in the manufacture of lead pipe. 
In Fig. 16 (scale J) A B is a thick scantling of wood, fastened ver- 
tically, to which a short cross-scantling, 0, is firmly braced by bolt- 
ing two boards, shaped as in the figure laterally against both A B 
and B G. B G carries a barrel of strong gas-pipe, a a a «, out of 
which the porcelain is to be pressed. To secure a a a a the piece G has 
been cut apart in the middle parallel to the plane of the paper, and the 
hole for the barrel is somewhat scant. Heuce when the two halves of G 
are drawn together by a couple of strong bolts the barrel is almost im- 
movably fixed. The barrel is surmounted by a cap, cc, through which 
passes a piston or plunger, d e, which can be moved up and down by 
the handle/// in the way which the figure readily shows. The lower 
end of the barrel is closed by the die.-cap h h. A lateral hole, b, allows 
of the introduction of porcelain or fire-clay slip until the barrel is quite 
filled. Downward motion of the plunger forces the slip through the 
1 Experiments since made with the crucibles, Fig. 14rt, showed that the vacuum 
boiling point of Bi is easily reached. To get a tight joint at e d, the space between 
lid and crucible is first calked with fibrous asbestos. A ring of fusible metal is then 
poured upon it. This melts at high temperatures ; but when the joint is well made, 
it is not forced through into the crucible. In future experiments I purpose to have 
the whole crucible made in a single piece. Cf. Preface. 
(749) 
