SEPTEMBER 3, 1897. ] 
known law of cooling. Not to mention 
minor workers, it was successively at- 
tacked and revived in most of the note- 
worthy high temperature investigations. 
Pouillet and Draper have studied it; Bec- 
querel, Crova, Violle, Le Chatelier, Lang- 
ley, Nichols, Paschen and others have ad- 
vaneed it. It remains to-day the most 
promising as well as puzzlingly fascinating 
subject for pyrometric research. One needs 
merely advert to its broad scope in relation 
to the temperature of the heavenly bodies 
to acknowledge this. Here I can only al- 
lude to Becquerel’s principle that the radi- 
ation of opaque bodies is spectrometrically 
alike at the same temperature, a result 
which has Crova’s more recent assent; to 
Violle’s photometric measurements of the 
total emission of platinum ; to the more re- 
cent work in the same direction of Violle 
and Le Chatelier, in which consistent re- 
sults were obtained for oxide of iron and 
platinum as far as 1500° to 1700°; to Ste- 
fan’s law as proved by Boltzmann, and the 
variety of discussion it has elicited ; to H. F. 
Weber’s collateral equation; to the Johns 
Hopkins measurements, etc. Another 
school of observers, including Langley, 
Paschen and others, has undertaken the 
promising but much more laborious method - 
of bolometric measurement of the distribu- 
tion of spectrum energy in its relation to 
temperature. Without doubt, however, the 
whole subject is yet i primis rudimentis; 
the results are confessedly ‘intrinsic.’ In- 
deed, vagueness in the nature of the radia- 
ting source lowers with sufficiently threaten- 
ing aspect to chill the fondest hopes. When 
que is told by Violle working on Mont 
Blane that the temperature of the sun is 
2500°; thereupon by Rossetti that it is 
- 9965°, by Le Chatelier that it is at least 
7600°, by Paschen that it is below 5000°, 
by Wilson and Gray that it is 8000°, etc., 
one wisely concludes that more may yet be 
learned about it. Oursympathies naturally 
SCIENCE. 
343 
go with those who, like Lummer and Wien 
and the Johns Hopkins people, are begin- 
ning fundamentally with the search for an 
absolutely black body. Less superstructure 
and more sub-cellar is, perhaps, the watch- 
word in radiation pyrometry. f 
Turning, now, to the last and most im- 
portant of the methods of practical pyrom- 
etry, we find a curiously meandering 
evolution apparent. I have already indi- 
cated that Pouillet (1836) was the first to 
complete a legitimate piece of calibration 
work. Pouillet might have condemned the 
method, but for some reason Tait’s thermo- 
electric anomalies of red-hot iron were not 
detected. Regnault (1847), who was the 
next to take up the subject as it happened 
with the same couple, made his condemna- 
tion sweeping enough. It was not the real 
perversity of the platinum iron couple 
which provoked Regnault, for of this 
neither he nor Pouillet became aware. 
Regnault’s objection (as we should put it 
to-day) lay in the fact that the thermo- 
couple obeyed Ohm’s law, which in that 
early day lay somewhat beyond the great 
physicist’s range of interest. Fortunately, 
but none the less long after, Becquerel fol- 
lowed with his palladium and divers plati- 
num couples, carefully calibrated and 
efficiently used. What these platinum 
couples were is not stated. They cannot 
have been very sensitive or they would 
have been preferred to the palladium-plati- 
num couple. Indeed, the metallurgy of 
platinum alloys did not reach a degree of 
refinement until Deville and Debray (1875) 
overhauled the chemical separation of 
platinum metals with particular reference, 
both to iridium and torhodium. Recently 
Mylius and Forster at the Reichsanstalt 
further contributed to platinum metallurgy. 
But in view of the toils in which the whole 
subject of high temperature measurement 
languished in Becquerel’s day, his results 
were not sufficient to remove the discredit 
