bakus.] METHODS OF PYROMETRY. 29 
pansion of cast-iron at high temperatures, availed themselves of bulbs 
of copper and of platinum for their thermal measurements. These were 
used much after the manner of vapor-density bulbs. The long capillary 
necks could be closed at the desired temperature by a faucet, and the 
temperature was then calculated from the amount of water which en- 
tered the cold bulb. These experiments form a natural transition 
to the earlier researches of Deville and Troost, 1 in which a splendid 
improvement in thermal measurements was made possible by the in- 
troduction of porcelain bulbs to replace those of metal and of glass. 
Deville and Troost here use Dumas's well-known method to evaluate 
both temperature and vapor density. In their search for a heavier 
thermal gas than air they select iodine vapor preferably to mercury, 
inasmuch as the metal is apt to condense on the colder parts of the 
bulb and in falling down upon the hot parts to cause fracture. Using 
this iodine thermometer, they find that cadmium and zinc boil at 860° 
and 1,040°, respectively. They also measure the coefficient of expan- 
sion of porcelain by noting the length of the necks of their bulbs at 
different (high) temperatures (0°, 860°, 1.000°). Having found these 
data they proceed to the measurement of vapor densities, with results 
which are not of interest here. 
The high values for the boiling point of zinc thus obtained conflicted 
very seriously with certain measurements subsequently made by Bec- 
querel. 2 This observer used a platinum-palladium thermo-couple, the 
indications of which had to be carefully referred to Pouillet's plati- 
num air thermometer. In this way Becquerel found the boiling point 
of zinc at 932°, more than 100° below that of Deville and Troost, as 
well as reaching a similarly low boiling point for cadmium, 746°. In 
the same paper the method of determining a series of melting points of 
metals is described and the data are fully given, and several final sec- 
tions are devoted to radiation pyrometry. As regards accuracy of 
measurement and varied character of results, this paper is one of the 
most important in the history of pyrometry. It is to be noticed that 
Becquerel was aware of the probable permeability of platinum to gases 
at high temperatures. Further mention will be made of this later. 
These discordant results necessarily provoked considerable discussion 
between Becquerel 3 and Deville and Troost, 4 which temporarily resulted 
in favor of the former. 
Deville and Troost naturally reject Becquerel's low values, and be- 
1 Deville et Troost : ISur la deusit6 de vapeur d'uu certain nombre de matieres rnin- 
erales; C. R., vol. 45, 1857, p. 821 (C. F. Berl. Ber., 1857. p. 73) ; C. R., vol. 49, 1859, p. 
239; Ann. ch. et phys., 3d series, vol. 58, 1860, p. 257. 
2 Becquerel: Recherches sur la determination des hautes temperatures. Ann. ch. 
et phys., 3d series, vol. 58, 1863, p. 49. 
3 Becquerel: C. R., vol. 57, 1863, p. 855; Inst., 1863, p. 369; C. R., vol. 57, pp. 902, 
925; Inst,, 1863, p 385. 
4 Deville et Troost: C. R., vol. 56, p. 977; Inst., 1863, p. 161; C. R., vol. 57, 1863, 
pp. 894, 935; Inst., 1863, p. 377; ibid., p. 897. 
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