340 
this place to the development of the only 
fruitful method of absolute pyrometry 
which has yet been devised. I refer, of 
course, to the gas thermometer, or, in other 
words, to the manometric methods of meas- 
uring the thermal expansion of gases. 
Efforts have, indeed, been made to use 
gaseous viscosity for absolute high tem- 
perature work. It has been definitely 
pointed out, inasmuch as viscosity in gases 
is independent of pressure, while both 
viscosity and volume increase with tem- 
perature, that the transpiration rates of 
gases through capillary tubes of platinum 
glazed externally would necessarily be 
an exceedingly sensitive criterion of the 
variation of high temperatures. The small 
volume of the transpiration pyrometer as 
compared with the clumsy fragile bulb and 
appurtenances of the air thermometer is 
further to the point. But modern kinetics 
has as yet failed to fathom the law of varia- 
tion of viscosity with temperature, and even 
the researches of O. E. Meyer in this direc- 
tion do not seem to have quite touched 
bottom. Gaseous transpiration pyrometry 
is thus still much in the air, surveying the 
horizon of a glorious future. 
Returning from this digression to the 
air thermometer, we find the first thorough- 
going piece of high temperature work car- 
ried out by Prinsep (1829), by the aid of 
a reservoir of pure gold to which I have 
already alluded. Prinsep’s manometer was 
filled with olive oil, and the volume issuing 
at constant pressures was found by the bal- 
ance. In view of the pure olive oil, proba- 
bly available in 1829, these experiments 
must have been comfortably appetizing, and 
I dare say Prinsep’s good humor in the 
matter may have contributed to the remark- 
able excellence of his results. Prinsep’s 
researches were not superseded until 
Pouillet, in 1836, published his paper on 
pyrometry. Pouillet constructed an air 
thermometer bulb of platinum and was 
SCIENCE. 
[N.S. Von. VI. No. 140. 
thus able to reach the farthest pyrometric 
north of the day and long after. His re- 
sults are many sided; they contain the first 
definite data in radiation pyrometry and in 
calometric pyrometry. His constant pres- 
sure manometer, afterwards further per- 
fected by Regnault, is the best apparatus 
for the purpose to-day. Pouillet did not 
suspect, indeed he remained quite unaware 
of, the permeability of platinum to furnace 
gases ; perhaps for this and other reasons 
he failed to detect the thermo-electric anom- 
alies in the platinum-iron couple which 
he so carefully calibrated. 
It was thus a great step in advance when 
Deville and Troost long after replaced plati- 
num by glazed porcelain, availing them- 
selves (1857-60) of Dumas’ famous vapor 
density method for measuring temperature. 
Unfortunately for the rapid progress of 
pyrometry, Deville and Troost used iodine 
vapor in their bulbs, a heavy gas indeed, 
but a gas, as was afterwards found, whose 
low temperature molecule dissociates at 
higher temperatures. Thus they unwit- 
tingly committed an even greater error 
than Pouillet in gliding over permeable 
platinum, and their data for the boiling 
points of zinc and of cadmium were about 
100° too high. In fact, these results were 
challenged not long after by Becquerel 
(1863), who had fallen heir to Pouillet’s 
platinum air thermometer, had used it to 
calibrate a platinum-palladium thermo- 
couple of his own, and had found data for the 
boiling points of zine and cadmium up- 
wards of 110° below those of Deville and 
Troost. I cannot here enter upon the dis- 
cussion which thereafter arose between 
these active observers further than to state 
that in the course of it both parties fre- 
quently repeated their measurements (Bec- 
querel even substituting a porcelain bulb 
for Pouillet’s thermometer) without remov- 
ing the decrepancy between their values. 
Later researches have decided in favor 
