46 MEASUREMENT OF HIGH TEMPERATURES. 
research of Fievez, 1 in which the attempt to associate the wave-length 
character of the spectrum with the temperature of the source has again 
been made. Eecently a number of German physicists have undertaken 
a re-interpretation of Draper's law. I have only space to allude to 
the papers of H. F. Weber, 2 Stenger, 3 and Kovesligethy, 4 by whom the 
questions relating to emission and absorption of light are being vigor- 
ously discussed. In America a series of well-known researches have 
been published by Laugley. 5 Having perfected the bolometer, and 
thus developed a new method for the measurement of radiant heat and 
light, Langley, in a series of researches which are still in progress, has 
determined the distribution of energy in prismatic solar spectra and in 
the spectrum of the grating. Proceeding thence to artificially incan- 
descent bodies, Langley is actively engaged in mapping out the char- 
acter of their spectra for all temperature 0° to 2,000° of the source. 
Less adapted for accurate measurement are certain pyrognomic sub- 
stances 6 which discolor with temperature, such as the iodides of copper 
and mercury, for instance. To this class belong the oxide coats 7 which 
form on iron and copper. The tints, however, depend not only on the 
temperature but very materially on the time of exposure. 8 
Viscosity. — Very little use has been made of the viscous qualities 
of a substance for temperature measurement. Sir William Thomson, 9 
indeed, proposed a thermoscope based on the change of viscosity of 
liquids, more especially of water with temperature; but it is intended 
for low temperatures, and does not seem ever to have been used. 
In my own work I have found that kaolins and fire clays could be 
classified by noting the amount of sag at a given temperature and for 
a given time, which rods of the same form and dimensions experienced 
when spanning the distance between supports at a given length apart. 
The criterion here is flexural viscosity. Such a process would lend 
itself for temperature measurement conducted in a way similar to the 
fusion experiments in the case of alloys. It seems curious, however, 
that no attempt has yet been made to base pyrometric measurements 
on the viscosity of gases. Following MaxwelPs 10 well-known investi- 
gation, the viscosity of gases is independent of the pressure and de- 
pendent only on the absolute temperature. From a theoretic point of 
view, therefore, such pyrometers have almost as much in their favor as 
1 Fievez : Bull. Acad. Roy. Belgique, 3d series, vol. 7, 1884, p. 348. 
2 H. P. Weber: Wied. Ann., vol. 32, 1887, p. 256. 
3 Stenger: Ibid., 1887, p. 271. 
4 Kovesligethy : Wied. Ann., vol. 32, 1887, p. 699. 
6 Langley: Am. Journ. Sci., 3d series, vol. 25, 1883, p. 169. 
6 Hess: Dingl. Jour., vol. 218, 1875, p. 183. 
7 Fischer: Dingler's Jour., vol. 225, 1877, p. 278. 
s Barns & Strouhal : Bull. U. S. Geol. Survey, No. 18. 
9 Thomson: Proc. Royal Soc, Edinburgh, vol. 10, 1880, p. 537. 
10 Maxwell: Pbilos. Mag., London, 4th series, vol. 19, I860, p, 19; ibid., vol. 32, 1866, 
p. 390; ibid., vol. 35, 1808, pp. 129, 185; Phi los. Trans., vol. 1, 1866. p. 249. 
(700) 
