IARUS.] 
PORCELAIN AIR THERMOMETRY. 
235 
jtemperature are distinguished. The table applies for e^ =11,000 micro- 
volts, the value found in Chapter II as the equivalent of T. 
Table 78. — Interpolations near the boiling point of zinc. 
Bulb. 
Series. 
T 
temperature 
increasing. 
temperature 
decreasing. 
Mean T. 
I 
I 
II 
II 
II 
II 
II 
IIIi 
IV 5 
"1 
III > 
IV | 
vj 
o 
912 
900 
925 
o 
915 
o 
£ 909.0 
1 
1 
- 918.8 
916 
924 
915 
914 
Hence, if to these mean values be added the corrections ST as just 
found, there follows T=921.5o for Bulb I and T=926.4o f or Bulb II. 
A final correction is still to be added. In the present reduction I 
assume e 20 =H,000 as the electrical equivalent of the boiling point of 
zinc. The careful crucible experiments made soon after the air ther- 
mometry, and detailed on page 123, show that e 20 =ll,074. Hence, since 
6T=6e 20 /{a+2bT) 
nearly, and since de 2 o=+71, it follows that 
ST = 4.7°. 
The constants a and b are calculated from the data of diagram (Fig. 42), 
a sufficient approximation. Hence, finally, 
T=926° for Bulb I, and 2=931° for Bulb II. 
This is the closest approximation to the boiling point of zinc which 
the method, in its present stage of development, permits. 
This is a low result as compared with Deville and Troost's, and with 
Weinhold's data. It agrees admirably with the values of Becquerel and 
of Violle. But my datum is not as low as would have been anticipated 
from the constant- volume measurements (page 199) of this chapter, or 
from inferences deduced by purely thermoelectric measurement in 
Chapter II. As long, however, as rigorous measurement of the vari- 
able /?, the coefficient of expansion of the bulb, has not been made, and 
the effect of a non-glazed interior of the bulb has not been placed 
beyond all obscurity, it is wise to advert to the present value for the 
boiling point of zinc with only precautionary emphasis. Nevertheless, 
it is pleasant to note this accordance of data between the results of the 
French observers mentioned and my own data. I am specially encour- 
(889) 
