172 MEASUREMENT OF HIGH TEMPERATURES. [bull. 54. 
They were made by Morlent freres, Paris 1 (formerly Mr. Gosse, the 
original constructor of the Deville and Troost apparatus), of the very 
refractory porcelain of Bayeux. Fig. 31 is the earliest form. Bulb and 
Fig. 31. Non-inglazed spherical air thermometer bulb. Scale \. 
stem are one piece, put together by the maker. The fractured bulbs 
show that the ballon proper, to within a radius of l cm of the neck [d d, 
Fig. 4), and the stem with attached neck (b d a d), were made separately, 
and then put together by a skilled artist. After burning no disconti- 
nuity of porcelain at the circle of junction is visible. The glazed gas- 
thermometer is perfectly smooth on the outside, and a longitudinal sec- 
tion differs in no essential respect from Fig. 31. 
In consequence of the long capillary stem (0.1 cm in diameter) it is 
exceedingly difficult to glaze these thermometers internally or to keep 
the stems from choking when the temperatures are high enough to 
soften the glaze. For this reason the bulbs are furnished without being 
glazed internally — an error when data of high temperature are to be 
sharply measured. I add here that on fracture the stems very fre- 
quently reveal clefts and lateral fissures communicating with the canal, 
I believe that a more compact stem could be made in the manner de- 
scribed above (p. 95) for the manufacture of porcelain insulators for the 
thermo-element. M. Gosse pressed his stems in a long mold over aj 
core of zinc wire. The latter is melted and volatilized during the firing ; 
but in spite of its ingenuity this method is imperfect, for the capillary 
canals made in this way, in addition to their liability to retain lateral 
fissures, are seldom perfectly central along the whole length of stem. 
In case of a stem made by my method this result necessarily follows 
whenever the apparatus has once been adjusted. 
In Table 43 some values of the mean dimensions, etc., of the bulbs (Fig. 
31) are given. They are the results of mercury or water calibrations, 
of which for accurate measurements the water calibrations are prefera- 
ble. Mercury does not so easily enter the fine fissures. 
1 1 also had similar bulbs made iu Berlin, and will communicate results obtained 
witb tbem later. 
(826) 
