232 MEASUREMENT OF HIGH TEMPERATURES. [bull. 64... 
the observed maximum error, apportioning tlie remaining four or five} 
degrees to the variety of discrepancies already enumerated, to which, 
may be further added changes both of v, the capacity of the bulb, and 
of /?, due to vitrification or similar progressive change of the substance] 
of the porcelain during successive heatings ; to irregular differences of 
the stem error; to the possible occurrence of minute capillary leakage! 
throughout the considerable length of connecting tube; to structural! 
changes (crystallization, silicification, gaseous corrosion) of the metali 
of the thermocouple. It is needless to make further mention. 
Considering the figures 41 and 42 as a whole, it will be seen that the^ 
calibration curves are regular throughout In case of the 20 per cent! 
platinum-indium alloy, therefore, no evidence of sudden allotropic] 
changes or polymerization is anywhere discernable. Hence betweenj 
300° and 1,300°, the availability of the given platinum-iridium alloy fog 
thermo-electric pyrometry can not be disputed. I do not believe thaw 
the strictures which Le Ohatelier (1. c.) has placed on the pyrometric usJ 
of the platinum-iridium alloys are substantiated by experiment, though] 
they may be true (Chap. I) for low percentage alloys. A full discussion! 
of the divergence of the said curves (Figs. 41, 42) from the Avenarius-j 
Tait equation, is beyond my present purpose. 
Accuracy of the measurements made, Group II — In a general way} 
these critical remarks apply to the data obtained with Bulb II and in-J 
serted in the chart (Fig. 42). The method of designation is clear, thel 
divers series beiug distinguished on the chart by dashes, which pass! 
upward or to the right for ascending temperatures, and pass downward! 
or to the left for descending temperatures. As a whole, the data forj 
Bulb II are a marked improvement upon the data for Bulb I. This was! 
brought about principally by correcting the calibrated volumeof thebulbj 
by such permanent changes of volume as occur after each heating. For-J 
tunately the value of this correction can be found with great accuracy! 
and facility by the same method by which the fiducial temperature isl 
determined (cf. page 213). If 6v be the permanent alteration of the vol-] 
ume of the bulb due to heating ; if b b and t h be the pressure and tern- j 
perature before heating, and b a and t a the pressure and temperature] 
after heating, for which, in each case, the air is wholly in the bulb andj 
capillary stems (i. e., 1^=0); and if 2(v) be the total original volume] 
of bulb and capillary stems, then 
b b f(t b )-b a f(t a ) _ 6v 
bj(t a ) 2v' 
The quantity bf(t) occurring in this equation is the same already evaki 
uated in equation 6 (page 213). The following tabular exhibit of the»;| 
values of 6v in question was obtained from Bulb II. 
(SS6) 
