52 MEASUREMENT OF HIGH TEMPERATURES. 
nary platinum, he does not attempt to formulate his data. A final and 
more ambitious attempt to express the resistance of temperature rela- 
tion is due to Oallendar. 1 Wishing to establish a strictly comparable 
standard of high temperature, he avails himself of pure platinum. This 
he compares directly with an air thermometer as far as 600°. A feature 
of the experiments is the inclosure of the wires within the bulbs of the 
air thermometer. Great care is taken to guard against surface conden- 
sation of gas. Data are tabulated. The work enjoys the supervision 
of J. J. Thomson. 
From a theoretical point of view the electrical conductivity of gases 
presents many-phases available for temperature measurement. Experi- 
ments on the conductivity of hot gases are due to Buchanan. 2 
I am aware that Mr. 0. A. Perkins, of Johns Hopkins University, has 
for some time been occupied with similar experiments. 
Magnetism. — A magnetic thermoscope was proposed by Thomson, 3 
but it is specially intended for low temperature. High temperature 
thermoscopes of this kind must obviously fail from the irregularity of 
the magnetic behavior of metals at high temperatures. 
Interpolation methods, — Methods of this kind are of the utmost service 
and have been much in use. In the case of a furnace, whose tempera- 
ture is increasing or decreasing regularly, a given unknown tempera- 
ture may be fixed between two known temperatures by time-inter- 
polation. The two or three known temperatures between which the 
unknown data lie may be convenient fusing points, sufficiently near 
together to make linear or quadratic interpolation practicable. Carnel- 
ley 4 and others have made much effective use of methods of this kind. 
ADVANTAGES OF THERMO-ELECTRIC PYROMETRY. 
Having thus indicated the chief methods of pyrometry so far em- 
ployed, it next behooves me to state clearly in what respect the thermo- 
electric method deserves preference before all others. To do this I must 
reiterate the point of view already emphasized in the preface and from 
which the greater part of the present volume has been written. It is 
my belief that before important steps in most subjects directly bearing 
on dynamical geology can be made, our methods of high temperature 
measurement and of high pressure measurement must first be facilitated. 
Moreover, the solutions to be given to the thermal and to the mechan- 
ical problems must be such that the high temperatures may be measured 
under conditions of high pressure, and conversely. 
When temperature measurements are to be made under these almost 
1 Callendar : Proc. Royal Soc. London, vol. 41, 1886, p. 231, 
2 Buchanan: Philos. Mag., London, vol. 24, 1887, p. 287. The subject is now being 
vigorously discussed; but there is no space for further references here. (Schuster, 
Blondlot, Elster u. Geitl, and others.) 
3 Thomson: Proc. Royal Soc. Edinburgh, vol. 10, 1880, p. 538. 
* Camelley : Jour. Chem. Soc. London, vol. 1, 1877, p. 365. 
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