barit.I METHODS OF PYRQMETRY. 37 
much as all these operations can be carried on under atmospheric 
pressure the apparatus was specially adapted for high temperature 
work. Messrs. Meyer 1 , utilizing these advantages, were able to obtain 
definite evidence on the probable dissociation of chlorine and iodine 
vapor. Crafts and Meier 2 then pointed out that Meyer's method could 
very readily be adapted for temperature measurement. Optic, calori- 
metric, and electrical methods of temperature measurement, they con- 
tend, are all dependent on the air thermometer, the results of which are 
reliable only in the case of very perfect mechanism, and are not available 
above the temperature at which porcelain is rigid. By inserting a 
capillary platinum tube into the neck of Meyer's apparatus the air can 
be lifted out by a current of carbonic acid gas or of hydrochloric acid 
gas, both of which are easily absorbed. It is possible to make vapor 
density measurement to alternate with thermal measurements; and 
since the operation may be completed in two minutes, absolute rigidity 
of the porcelain vessel is not rigorously essential. Entering into the 
spirit of this suggestion, Meyer 3 and his pupils opened a new field of 
pyrochemical research, in which, after establishing the constancy of 
the coefficient of expansion of permanent gases at high temperatures, 
they extend their inquiries further to vapors. Meyer's apparatus here 
is a hollow sphere of porcelain provided with axial capillary tubes for 
influx and efflux of gas. 
Following Meyer's summary, the linear character of the heat expan- 
sion of gases at high temperatures (barring dissociation) is to be 
regarded as established for selenium and tellurium (Deville and Troost), 
for nitrogen, oxygen, mercury vapor, and As 2 G 3 vapor (V. and C. 
Meyer), for hydrochloric acid, and carbonic acid gas (Grafts), and for 
hydrogen (Meyer and Ziiblin). These inferences antagonize the pub- 
lished opinion of Troost, 4 who, with Berthelot's acquiescence, prefers to 
regard the expansion of gases at high temperature (iodine for instance) 
as a physical function of temperature rather than to accept the occur- 
rence of dissociation.' 
Meyer 5 and his pupils, however, push their investigations into much 
greater detail, adding to the number of gases of constant thermal c >- 
efficient and interpreting the variable behavior of others. Their nv w 
researches are carried on at temperatures even as high as 1,700°. Their 
apparatus is a long platinum tube provided with terminal capillary 
stems of platinum which have been ground into the somewhat narrow it 
ends of the tube. This thermometric tube is surrounded by fire-clay 
which in its turn is enveloped by a second and wider platinum tube. 
l V. u. C. Meyer: Berl. Ber., vol. 12, 1879, p. 1426. 
2 Crafts and Meier: C. R., vol. 90, 1880, p. 606. 
3 Meyer: Berl. Ber., vol. 13, 1880, p. 2019; ibid., vol. 15, 1882, p. 1161. 
4 Troost: C. R., vol. 91, 1880, p. 54. 
5 Lauger u. Meyer: Pyrochemi.sche Untersuchungen, Braunschweig, Vieweg u. 
Bonn, 1885; Berl. Ber., vol. 18, 1885, p. 1501. • 
(691) 
