30 MEASUREMENT OF HIGH TEMPERATURES. 
lieve them to be erroneous because of the permeability of platinum at 
high temperatures. In doing this they refer to researches of their own 1 
on the porosity of metals. Becquerel's reply is of an experimental 
character. He continues his work on air-thermometer pyrometry, re- 
placing the platinum bulb with bulbs of porcelain, and availing himself 
both of constant pressure and of constant volume methods of measure- 
ment. Curiously enough the results of these new determinations are 
even below the former values, the boiling points of zinc and of cadmium 
being at 891° and 720°, respectively, while for the former as low a value 
as 884° was found. Becquerel dwells upon the excellence of the Pou- 
illet method lor high temperatures. * Deville and Troost nevertheless 
refuse to regard these new results of Becquerel's as conclusive. They 
insist upon the impossibility of deriving accurate data with a porous 
reservoir. They point out that the large difference between BecquerePs 
present and former results is in itself to be looked upon with suspicion. 
They finally assert, inasmuch as BecquerePs pyrometers were not in 
immediate contact with zinc vapor, but were exposed in a closed lateral 
tube which issued from the zinc retort, that the datum measured is not 
the boiling point of zinc but a temperature below it. They finally re- 
peat their own experiments with the same values as before. Becquerel 
again endeavors to show that the permeability of platinum did not se- 
riously influence his results. He shows that his own researches are 
made in a way calling for much less skilled manipulation than those of 
Deville and Troost; and he finally adds that Deville aud Troost have 
made but a single measurement with air, and that the use of iodine 
vapor as a gas for thermal measurement is not immediately warranted. 
Becquerel states the reasons for considering his boiling-point apparatus 
sufficient, but agrees that a possible error may be the impurity of his 
zinc. With these remarks discussion ended, being left without a final 
issue; but it is well to state, in passing, that the results of subsequent 
observers, including Deville and Troost themselves, have proved be- 
yond a doubt that the later inferences of BecquerePs were very nearly 
correct. Victor Meyer, 2 I believe, was the first to suggest the possible 
dissociation of the iodine molecule at high temperatures, a behavior 
which he had established for chlorine. Meyer's views were corrobo- 
rated and variously interpreted by Crafts and Meier, 3 by V. Meyer him- 
self, 4 Crafts, 5 Troost, 6 Berthelot, 7 and others. 8 
1 Deville and Troost: Porosite du platine. R£p. chim. appl., 1863, p. 326; sur la 
permeabilit6 du fer a haute temperature ; C. R., vol. 57, 1863, p. 965. 
2 V. and C. Meyer: Berl. Ber., vol. 12, 1871), p. 1426. 
3 Crafts and Meier: C. R., vol. 90, 1880, p. 606; Berl. Ber., vol. 13, 1880, p. 851. 
"V. Meyer: Berl. Ber., vol. 13, 1880, p. 391; ibid., 1880, p. 1010. 
5 Crafts: Ibid., 1880, p. 1316. 
6 Troost: C. R., vol. 91, 1880, p. 54. 
7 Berthelot: ibid., p. 77. 
8 Cf. Deeriug; Chem. News, London, vol. 40, 1879, p. 87. 
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