barus.1 METHODS OF PYROMETRY. 45 
beli finds tbat the law of Stefan very closely interprets his experi- 
ments. In a series of beautiful experiments Schleiertnacher ! puts the 
same law to a rigorous test. He heats a platinum wire to incandes- 
cence in an inclosure, the walls of which can be heated to different 
constant temperatures (0° to 200°), and from which all air has been 
carefully exhausted. The actual temperature of the wire is calculated 
from its resistance, a series of subsidiary researches in which the wire 
is compared with the porcelain air thermometer having previously been 
made. The amount of heat generated in the wire following from Joule's 
law, Schleiermacher has the data necessary to test Stefan's law. In, 
this way he proves that the heat emitted from platinum, covered or not 
with copper oxide, increases with temperature in greater rate than 
Stefan's law predicts. Schleiermacher then interprets the discrepan- 
cies observed. 
At the close of the present paragraph a few references to the use of 
the radiation pyrometer for evaluating solar temperature and others 
of great intensity is in place. Passing over the earlier measurements 
we find a paper of Soret, 2 and at about the same time one by Violle. 3 
The latter's memoir is particularly complete, containing the history 
of the subject and a discussion of methods and apparatus. Making 
his observations on Mount Blanc, Violle finds 2,500° for the surface tem- 
perature of the sun. After this, observations on the temperature of 
flames, of the electric arc, and of the sun, were published by Rossetti. 4 
Using a thermo-couple, he investigates a law of radiation as far as 
the boiling point of mercury, which law he carefully formulates. With 
due allowance for atmospheric absorption, Rossetti finds 9,965° as the 
sun's surface temperature. The process is, of course, one of extrapola- 
tion. The same method applied to the temperature of the electric arc, 
gives 2,500° and 3,900° as the temperatures of the negative and posi- 
tive poles respectively. A further important contribution to solar sur- 
face temperatures is due to Crova. 5 
Other optic methods of pyrometry 6 endeavor to establish the rela- 
tions between temperature and the character of the spectrum. Dewar 
and Gladstone 7 attempted and finally abandoned a project of this kind. 
On the other hand, however, Stas 8 refers with some enthusiam to the 
•Schleiermacher: Wied. Ann., vol.26, 1885, p. 287. 
2 Soret: Ann. de l'^cole norm. sup6r., 2d series, vol. 3, 1874, p. 435. 
3 Violle: Ann. ch. et phys., Paris, vol. 10, 1877, p. 289. 
"Rossetti : Ann. ch. et phys., vol. 17, p. 177 r 1879 ; C. R., vol. 89, 1879, pp. 384, 781 ; 
Philos. Mag., London, 5th series, vol. 8, 1879, p. 324. 
5 Crova: C. R., vol. 95, 1882, p. 1271. 
6 Prof. Cleveland Abbe has called my attention to a paper in the Comptes Rendus, 
in which the continuous change of rotation of the plane of polarization of quartz, 
with temperature, is made the basis of thermal measurement. Unfortunately I can 
not now supply the reference. 
7 Dewar and Gladstone: Chemical News, vol. 28, 1873, p. 174. 
8 Stas: Bull. Acad. Roy. Belgique, 3d series, vol. 7, 1884, p. 290. 
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