34 MEASUREMENT OF HIGH TEMPERATURES. 
500 atmospheres and for temperatures below 300°, and in the hands of 
Arnagat they led to the discovery of minima of " pv." These researches, 
which must be passed over briefly here, are not as yet in a state of prog- 
ress to enable the results to be at once applied. It is therefore reassur- 
ing to find iu the labors of Langer and Meyer 1 a number of data tending 
to show that at high temperatures and for moderate pressures the con- 
stancy of the co-efficient of expansion of gases may be warrantably 
assumed. 
To return from this digression to the subject in hand I find an im- 
portant research by Erhard and Schertel 2 in which the melting points 
of Priusep's alloys are again carefully determined by the porcelain air 
thermometer. The bulbs are of Meissen porcelain, and the method of 
measurement is essentially that of Weinhold. Finally, in 1880, De- 
ville and Troost 3 publish a succinct account of their results in high 
temperature measurement, and thus conclude the interval of compara- 
tive silence. 4 They describe a new form of air thermometer, apparently 
superior to the Begnault normal form. In this instrument the air of the 
bulb is transferred into the measuring apparatus by a SprengeFs pump. 
The bulb itself, being placed in a furnace fed by heavy petroleum oil, 
can be heated to any desired temperature by supplying a greater or 
smaller ainount-of fuel, through a graduated stop-cock. To eliminate 
the stem error they again use the "compensator," which is a closed 
porcelain capillary tube identical with the stem of the air thermometer 
and exposed side by side with it. This compensator is provided with 
its own manometric attachment. Nitrogen is used preferably to air. 
The last memoir contains a full digest of their results on the boiling 
point of zinc. The methods of experiment and of measurement are also 
tersely given in chronological sequence. The authors put great stress 
on the purity of their zinc, on the fact that no iron was used in the re- 
torts, on the great mass of zinc distilled (17 kg. to 20 kg.), on their 
methods of protecting their bulb from direct radiation by multiple 
screens, and on the great heat of the circumambient flame. The porce- 
lain bulb, its peculiarities, and its construction are described with some 
detail. Their mean value for the boiling point of zinc, as Troost 5 sub- . 
sequently remarks, is 942°, and the number of measurements made, 27. 
In some experiments made at a later date by Troost 6 the boiling point j 
of selenium was found between 664° and 683°, the determination being 
feasible in a vessel of enameled iron. Troost 7 therefore concludes that 
1 Langer u. Meyer : Pyrochemische Untersuchungen, 1885. 
2 Erhard and Schertel: JahrbuchfurdasBerg-und-Hiittenwesen, ini Konigr. Sach 
sen, 1879, p. 154. 
3 Deville et Troost: C. R., vol. 90, 1880, pp. 727,773. 
4 Deville: C. R., vol. 74, 187a, p. 145; is speculative, and refers to excessively hig] 
and to vsolar temperatures. 
r ' Troost : C. R., vol. 94, 1882, p. 788. 
6 Troost: Ibid., p. 1508. 
7 Troost: Ibid., vol. 95, 1882, p. 30. 
(688) 
