3AEUS.I PORCELAIN AIR THERMOMETRY. 169 
rhe end in question projects somewhat beyond the lower edge of the 
sap, the inner width of which is such as to fit snugly around the stem 
of the porcelain air thermometer P. Capillary tube and stem are 
fastened with resinous cement 1 in this manner: The cap is clamped ver- 
tically with the open end uppermost in a vise and filled with melted ce- 
ment, kept slightly above the melting point by applying a burner. The 
stem of the air thermometer is then inserted from above and in such a 
way that the projecting capillary tube may pass into the capillary canal 
of the stem. The cement which exudes is not removed until the two 
parts have thoroughly cooled in the upright position in which they were 
clutched by the vise. 
Metallic capillary tubes. — The capillary tube being some two meters 
long, it is convenient, or perhaps even essential for quick manipulation, 
to insert an intermediate branch stop-cock of glass, L K, somewhere 
near the middle of the tube. By connecting this cock with the exhaust 
pipe R 8 of a mercury air-pump, the air of the thermometer bulb may 
be removed in any quantity, the bulb thoroughly dried at red heat, or 
Bven filled with other gases than air, in a way which is thoroughly satis- 
factory. To insert the ends of the capillary tubes into the cock the 
following method is reliable: The iuner ends of the capillary tubes are 
bent at right angles to their axes, and then passed through the two 
parallel canals of a little piece of the porcelain cylinder used above for 
insulating the wires of the thermo-element. The parts near the right 
angles are additionally secured by being tied together with fine copper 
wire, whereupon the whole connection (excepting, of course, the ends 
of the capillary canals of the metal tubes, which, for this purpose, pro- 
ject beyond the porcelain cylinder) is saturated with cement. The 
cylinder is then inserted into one tube of the glass faucet and sealed 
in it with cement in such a way as to leave a minimum of waste space 
between the cylinder and the plug of the faucet. It is well, moreover, 
to tie the tubes proper to a stick, so as to bridge over the severed part 
Z, and thus avoid those leaks to which the joint is liable when suddenly 
jerked or otherwise interfered with seriously. It is convenient to make 
the exhaust-pipe E S of thin lead tubing, because this can be bent in all 
directions and is sufficiently rigid to maintain its figure and sufficiently 
impervious to gas. Two metallic capillary tubes may be coupled by 
pushing them in opposite directions into a short end of glass tubing, 
into which they fit snugly, till the ends meet near the center. Applying 
ja drop of melted cement and the burner to the end of the glass tube, 
the cement is drawn into the space between the outer surface of the 
inetallic tube and the inner surface of the glass tube by capillary at- 
traction, so long as the cement remains liquid. This joint is excellent. 
1 Rosin and beeswax fused together in equal parts. In soldering a wire or capil- 
ary tube axially into a large glass tube tbis cement must be added in thin horizon- 
tal layers, otherwise the cylinder of mastic, as it contracts on solidifying, draws itself 
iway from the tube. Marine glue is doubtless a more reliable cement. 
(823) 
