barus.] METHODS OF PYROMETRY. , 31 
Iii 1863 Deville and Troost 1 began the publication of another series of 
investigations on high temperatures and boiling points. They describe 
their new porcelain air thermometer bulb, which is a hollow sphere of 
porcelain, glazed both within and without, with a short neck, to which 
a capillary fissureless porcelain stem is soldered with feldspar, and the 
oxvhydrogen blowpipe. They propose to discard iodine and to use air 
in its place, giving their apparatus a form nearly identical with 
Regnault's 2 normal air thermometer. They insist on the importance of 
spherical bulbs, and the air contained is dried at red heat by aid of a 
vacuum pump. Ail the zinc is carefully purified, and used in large 
quantities (charges of 17 kg.). It is but just to add here, to the great 
credit of Deville and Troost, that the actual construction of the porce- 
lain air thermometer occupied them for nearly seven years, working in 
concert with M. Gosse, in charge of the porcelain works at Bayeux. 
They were the first to use metallic vapor baths for constant high tem- 
peratures, a method which has been adopted by Becquerel and by 
physicists generally since that time. In 1864 Deville and Troost 3 pro- 
ceeded toward the accurate measurement of the heat expansion of the 
Bayeux porcelain. Using a porcelain bulb simultaneously and of the 
same material as the porcelain of the dilatation apparatus, they have 
the data sufficient to eliminate the error due to heat expansion from the 
thermal measurements made. Their method is necessarily one in which 
the linear expansion of a porcelain rod exposed in a zone of known 
constant temperature is measured by the cathetometer. Two platinum 
buttons, inserted in the ends of the stem, subserve the purpose of 
fiducial marks, and they are viewed through long lateral porcelain 
sight-tubes in the constant temperature apparatus. In this way they 
show that in some 200 measurements the cubical expansion of porcelain, 
between 0° and 1,500°, is 0.000016 to 0.000017. Above 1,500° it be- 
comes rapidly larger. In addition to this normal heat expansion, 
porcelain experiences permanent dilatation, as is proved both by 
measuring the linear dimensions aud by density tests applied to the 
porcelain after heating. Curiously enough, this density diminishes 
with frequent heating. The permanent expansion, which is a very 
serious error in the first heating (the volume of a bulb increasing from 
281.3 CC to 285.6 CC in six heatings, for instance), fortunately, soon be 
comes negligible. Deville and Troost, at the end of their work, justly 
congratulate themselves on these results : " Nous conclurons que la por- 
celaine de Bayeux, matiere absolument impermeable et encore rigide 
aux 1,500° * * * capable de se dilater jusque la d'une maniere uni- 
forme, sans qu'on ait a tenir compte de sa dilatation permanente si ce 
n'est au but des experiences." They again emphasize the excellence of 
soldering together, with feldspar in the oxy hydrogen flame, the accu- 
rately calibrated bulb and stem. 
1 Deville aud Troost, vol. 57, 1863, p. 897. 
2 Regnault : loc. cit., PI. I, Figs. 7, et seq. 
3 Deville and Troost: C. R., vol. 59. 1864, p. 162, 
