barus.) METHODS OF PYROMETRY. 39 
porcelain bulb. Weinhold (loc. cit.) who examined this apparatus con- 
demns it, at least so far as the u pyrometre a marbre" is concerned. It 
appears that the carbon dioxide emitted is not again absorbed with 
sufficient regularity to subserve the purpose of thermal measurement. 
Perhaps Troost's 1 diffusion method for studying high temperature dis- 
sociation is to be added to this paragraph. 
Fusion. — These pyrometers are discontinuous as well as intrinsic. 
Nevertheless, in virtue of their simplicity they are among the most serv- 
iceable of all the forms of pyrometers devised. As long ago as 1828, Prin- 
sep, 2 using an air-thermonietor bulb of gold, endeavored to measure the 
melting points of silver-gold, silvej-platinum, and gold-platinum alloys. 
The brothers Appolt 3 investigated similar data for copper-tin alloys, 
using a calorimetric thermometer for high temperature measurement. 
A special double crucible for fusion of silver-platinum alloys is given by 
Heeren. 4 Temperatures estimated by alloy fusion were largely made use 
of by Plattner. Becquerel, 5 in his extended paper on the measurement 
of high temperatures, gave considerable attention to melting points. 
He used metallic wires and measured the fusing temperature with his 
calibrated thermo-couple. After him, Eiemsdyk 6 made a series of meas- 
urements on metallic melting points. A very ingenious series of ring- 
shaped cups, placed on a common axis in a tier, was suggested by 
Heeren. 7 These cups contain rings of alloy, the consecutively varying 
melting points of which are stamped on the bottom of the cups. After 
each observation the rings are simply turned. Carnelley 8 made use of 
fusion pyrometers, substances of known melting points being inclosed 
in capillary tubes to serve for the identification of similarly exposed 
substances of unknown melting point. Results on the melting points 
of platinum alloys are due to Roberts. 9 A more elaborate series of re- 
searches is due to Yiolle, 10 whose data for high melting points are pre- 
sumably the best in hand. Violle makes a careful study of the relation 
between specific heat and temperature. Assuming this relation to hold 
as far as the melting point in each case, he fixes this point for silver 
(954°), gold (1,035°), copper (1,054°), palladium (1,500°), platinum 
(1,775°), and iridium (1,950°), calorimetrically. For metals which melt 
below the platinum point, either the metal itself or platinum may be 
iTroost: C.R.,vol. 89, 1879, p. 306. 
2 Prinsep: Trans. Royal Soc, London, 1827; Ann. ch. et phys., 2d series, vol. 41, 
1829, p. 247; Pogg. Ann., vol. 13, 1828, p. 576; vol. 14, 1828, p. 529. 
3 Mitth. des Gewerbe Vereins fur Hanover, 1855, p. 345. 
"Heeren: Dingler's Jour., vol. 161, 1861, p. 105. 
5 Becquerel: Ann. ch. et phys., 3d. series, vol. 68, 1863, p. 49. 
6 Riemsdyk: Jahresber. d. Chem., 1869, p. 993; Chein. News, London, vol. 20, 1869, 
p. 32. 
7 Heeren: Zeitschr. des Vereins d. Ingenieure, 1876, p. 314. 
8 Carnelley: Jour. Chem. Soc, London, vol. 33, 1878, p. 281. 
9 Roberts: Ann. ch. et phys., 5th series, vol. 13, 1878, p. 111. 
10 Violle: C. R., vol. 85, 1877, p. 543; vol. 87, 1878, p. 981 ; vol. 89, 1879, p. 702. 
(G93) 
