36 MEASUREMENT OF HIGH TEMPERATURES. 
pended at one end and heated to extreme whiteness, it is probable that 
some permanent elongation will occur by virtue of the viscosity of the 
hot rod. The question therefore occurs whether this permanent expan- 
sion may not to some extent have produced the dilatation due to vitri- 
fication whicb they observed, or have been partially confounded with 
it. Messrs. Deville and Troost were careful to test the specific gravity 
of their heated porcelain, and they found a dimunition of density, a re- 
sult in harmony with the dilatation observed. Again, the fact that per- 
manent expansiou vanished after successive heating in their experi- 
ments is evidence in their favor. 
And yet I regard this remark not superfluous, because, in my own 
experiments, in which careful volumenometric tests of the volume of 
the bulb after successive heatings to 1,000° or 1,200° were made, I ob- 
served no permanent dilatation of volume. The increments I found 
would rather point to contraction. It does not seem probable, more- 
over, that porcelain which has been thoroughly fired in the manufact- 
urer's furnace would continue to change in volume for some time after, 
at a temperature at which porcelain is appreciably viscous. 
Dilatation of gases {displacement methods). — The next important step 
in air thermometry was made in Germany by V. Meyer and his pupils, 
although an ingenious suggestion, which was probably the main in- 
centive to those researches, is due to the American, Crafts. So far as I 
have read, Regnault 1 appears to have been the originator of methods 
of air thermometry in which the thermal gas, instead of being measured 
manometrically, is chased out or displaced by a second gas, which can 
subsequently be absorbed or otherwise eliminated. Eegnault's bulb 
is a large cylinder of iron provided with two capillary stems adjusted 
axially, and adapted for the admission and efflux of the gases. He 
bases his measurements on hydrogen, which is chased out, when the 
desired temperature is reached, by oxygen. Oxygen burns the hydro- 
gen, and it is therefore necessary only to absorb and weigh the water 
thus produced. It is obvious that this operation may be repeated as 
often as is desirable. 
Schinz (loc. cit.), who repeated these measurements, did not find them 
satisfactorily accurate. Nevertheless, a process which is of inferior 
accuracy below 1,500° may be very serviceable above the temperature 
at which the ordinary methods fail, and porcelain becomes viscous or 
even liquid. This appears to be the case with the displacement method 
as Crafts pointed out. 
In 1878 Victor and C. Meyer 2 published an account of a new method 
of determining vapor density. In this the air contained was mechan- 
ically lifted or chased out at any given temperature by vapors issuing 
from the substance whose vapor density was to be determined. Inas- 
1 Regnault: Ann. cb. et phys., 3d scries, vol. 63, 1861, p. 39. 
2 V. Meyer: Berl. Ber., vol. 11, 1878, p. 1867; V. u. C. Meyer- Ibid., 1878, p. 2253; 
Dingier'** Jour., vol. 231, 1878, p. 330; vol. 232, 1879, p. 418, 
(690) ' 
