344 
which Regnault had thrown upon thermo- 
electric pyrometry. And so it happened 
that the return to the method in recent 
date was of the nature of a resuscitation. 
It is amusing to note, as we pass on, the 
pranks of custom as it bore down upon 
pyrometry. Following Deville and Troost, 
every worker (I might mention at least 
five) felt in duty bound to redetermine 
the boiling point of zinc—rather a difficult 
feat in its way. Thus we find boiling zinc 
inseparably associated with the destiny of 
the calibrated thermo-couple. Le Chatelier 
broke this law of fateful sequence by igno- 
ring the need of calibration at the outset, 
and then using the couple so dignified to 
determine the melting points of silver, gold, 
palladium and platinum. But these are 
Violle’s melting points. Hence the py- 
rometric feature of Le Chatelier’s platinum- 
rhodium couple was in its inception due to 
Violle. 
Meanwhile, accompanying the geologic 
inquiries of Clarence King, an extensive 
series of pyrometric investigations which 
had been in progress in this country since 
1882 were completed (1887). These con- 
tained a full examination of divers efficient 
methods of pyrometry and a study of the 
porcelain air-thermometer with particular 
reference to the calibration of thermo- 
couples. In the course of this work the 
admirable pyrometric qualities of the plati- 
num iridium alloy were exhibited by de- 
tailed and direct comparisons with the air 
thermometer. It was shown that the cali- 
bration could be made permanent by refer- 
ring the thermo-electromotive forces to a 
Clark’s cell; that the character of their 
variation with temperature is uniformly 
regular, and that the thermal sensitiveness 
of the couples increases as the higher de- 
grees of red and white heat are approached. 
Finally, it was pointed out that couples 
destroyed by silicate corrosion or in similar 
ways could be restored by fusing over again 
SCIENCE. 
[N. 8S. Vou. VI. No. 140. 
on the lime hearth with merely negligible 
changes of constants. Elsewhere, Le Chate- 
lier’s clever combination of the platinum 
rhodium couple with the D’Arsonval 
galvanometer, then a comparatively new 
instrument in the laboratory, secured im- 
mediate favor. Professor Roberts-Austen, 
ever on the watch to waft good things across 
the channel from Gaul into Albion, hailed 
the new comer with no uncertain sound. 
Sometime after, the platinum-rhodium 
couple entered Germany and was there de- 
finitely calibrated (1892) for the first time, 
as already stated, at the Reichsanstalt. 
Of the three available couples, palladium, 
platinum-rhodium and platinum-iridium, 
the former is excluded from competition by 
reason of its low fusibility. Between plati- 
num-iridium and platinum-rhodium, the 
latter has been more extensively advertised, 
but is otherwise inferior to the older plati- 
num-iridium alloy. In other words, plati- 
num-iridium, when suitably alloyed, can 
be made more sensitive than platinum-rho- 
dium in the ratio 100 to 76. Beyond this 
the alloys are much alike ; both are tena- 
cious, resilient, refractory metals, and their 
thermo-electric forces under like conditions 
of temperature show a constant ratio even 
at extreme white heats. The thermo-elec- 
tric activity of these two alloys is exceed- 
ingly remarkable. Among over fifty differ- 
ent platinum alloys examined no similarly 
sensitive combinations were found. More- 
over, whereas platinum alloys of extremely 
large electrical resistance are not unusual, 
such metals are not apt to be distinguished 
thermo-electrically. 
To conclude: the small dimensions of 
the sensitive point of the thermo-couple, 
the independence of the intermediate tem- 
peratures between the junctions (apart 
from small corrections due to thé Thomson 
effect) and therefore the removal of the 
terminal difficulty, the high upper limit of 
the measurable temperatures, the perma- 
