178 MEASUREMENT OF HIGH TEMPERATURES. [bull! 
It is perhaps best to commence the heating with the lime furnace r< 
moved, sliding it above the neck. When the parts are white hot an 
the frothing has largely ceased, it is expedient to conduct the fur the 
soldering by hand directly, the blow-pipe being for this purposj 
manipulated by the right hand and the spindle turned suitably wit ; 
the left. Care must betaken not to melt the porcelain. The heat m 
however, sufficient to make porcelain quite viscous, aud not only cay 
the stem be bent, but the parts of the neck of the bulb surrounding thi 
lower end of the stem may be pressed firmly against it, producing 
weld joint, as it were. I have no doubt that porcelain can actually bi 
welded in this way. Flat steel pliers, which if necessary may be notche 
by a cylindrical hole which tits closely around the neck, are closer 
quickly but firmly around the neck and then quickly withdrawn, ro» 
tion of the bulb being temporarily discontinued. The operation call 
for skilled manipulation. Indeed, it is not easy to make a vacuum 
proof joint, and samples neat in external appearance have frequent!,! 
to be duplicated. It is essential to keep the whole neck at white heq; 
a long time in order that its inner surface and the outer surface of th 
stem may be everywhere in contact. It is at this stage of the operation 
that the lime furnace may be appropriately lowered, and the finli 
gradual coalescence of contiguous parts of the porcelain apparatii 
allowed to take place. Two oxyhydrogen flames impinging on the paji 
celain from opposite directions are preferable to a single flame with tql 
lime furnace. The latter is essential, however, during cooling. Poi 
celain is specially liable to crack on cooling when it first becomes rigid 
Hence it is expedient, after withdrawing the flame, to close up the fui 
nace as far as possible with asbestus board and carded asbestus, anj 
then to bury the whole furnace above the plate mm in a heap of slackej 
lime. But with the best precautions the feldspar is found to be fissure* 
after cooling, and unless bulb and stem have thoroughly coalesced ani 
the joint be perfect at the internal faces, the apparatus will not tj 
vacuum-proof. It is well to fuse feldspar upon the end of the stei 
around the hole. To keep this hole opposite the capillary hole in thj 
neck a thick platinum tube or wire may be thrust through both. Ij 
this case the neck of the bulb may be quite filled with the slimy felq 
spar paste, and the stem then forced into it from above, keeping tlu 
capillary platinum tube in place. The feldspar which exudes is pal 
tially removed, and the bulb then allowed to dry. This process givd 
greater assurance that soldering will take place between the inner sin 
face of the neck and the end of the stem than any other. I may add, il 
closing, that I have repeatedly tried to make air thermometers in whic) 
a capillary platinum tube is soldered into the stem with a glaze rnorj 
fusible than feldspar. I have also attempted to glaze single-piece aj 
thermometers, like Fig. 31, internally, as well as to solder platinua 
tubes into the stem. Although these attempts have thus far failed, 1 
is but just to assert that the failures are due rather to the insufficient 
(S32) 
