Measurement of High Temperatures. 49 
De la Provostaye and Desain* give as the reflecting power of cold 
platinum for lamp light (unpolarized), 0°677, so that for this 
metal when cold, A == 0°328. 
The difference between these values is so large, that making all 
reasonable allowance for the inaccuracies of both researches it 
seems certain that platinum at 1650° has a much greater absorp- 
tive power for the rays of the visible spectrum than at ordinary 
temperatures, 
his experiment shows that platinum at least, of those sub- 
stances affording metallic reflexion, offers no exception to the 
eneral law deduced for other bodies, viz: that A and E are 
unctions of the temperature. The nature of this function, and 
the dependence of A and E upon the wave-lengths of the rays 
in question must be made the subject of special and extended 
investigation. It is an unexplored domain. Almost the only 
researches which give substance for a probable surmise, are 
those of Jacques already mentioned. The fact that the position 
in the spectrum of the maximum of thermal intensity is a func- 
tion of the nature of the glowing substance and independent of 
its temperature, points to the conclusion that solid bodies belong 
to the first three classes. 
<a A must be experimentally determined, or failing in 
this, its values found for the various glowing bodies it is most 
desired to measure. It will then only remain to subject the 
results in Table [X (Paper I) toa further reduction so that they 
may be made to express the effect of temperature upon the rays 
from an ideal “ black body” instead of those emitted by glowing 
platinum; and finally to obtain a satisfactory comparison o: 
the platinum thermometer with the scale of the Centigrade air 
thermometer. Experiments to this end are in preparation by 
the author. 
Peekskill, New York, July 1, 1879. 
_* De la Provostaye et Desain, Comptes Rendus, xxxi, p. 512; also, Annales de 
Chimie et de Physique, III, xxx, 276. 
Am. Jour. Sci.--THrrD anaes Vou. XTX, No. 109.—Jan., 1880. 
