VRUS.l 
PORCELAIN AIR THERMOMETRY. 
199 
The results of this table are startling. They seem to show that bulbs 
ot glazed internally are worthless for temperature measurement. The 
rror is very largely due to the presence of moisture in the bulbs; prob- 
bly also to the condensation of air in the pores of the unglazed porce- 
lin. An explanation of these results can be given by equation (13) 
nd Table 48. This consideration exhibits an important principle, so 
ir as drying by exhaustion is concerned, and may therefore be made 
ere. Bulb No. 4, which is most carefully exhausted, need alone be 
iscussed. It appears that the value of 3 H and 3 h , which give rise 
T 
an error of .^ are approximately 3 H = 2.7 cm , and 3 h () = l.S' '". 
low, after a mercury air-pump has .been much used for drying, moist- 
re is apt to show itself in the receiver, after which the exhaustion can 
ot be carried further than 2 cm at ordinary temperatures. Hence it 
ppears that unless the exhaustion be very frequently repeated a ten- 
ion of aqueous vapor equivalent to about 2 cm of mercury at 100° may 
til to be removed, an amount nearly sufficient for the discrepancy in 
uestion. It is to be noted that like errors in H and h do not compen- 
ite each other. The amount of water thus remaining in the pores of 
le cold bulb is about 4 m s. 
These results point out the desirability of perfect exhaustion and 
ie necessity of keeping the air-pump dry. In view of the enormous 
iscrepancies cited in the last table, the unglazed bulbs were discarded, 
ome years after, however, when the difficulty of soldering porcelain 
ad been tried in many experiments, I resolved to test these unglazed 
nibs again, with a view to perfecting them. In the first place it 
obvious that if, instead of determining h directly with melting 
aow, this datum be calculated from measurements of h made at ordi- 
ary temperatures (25°), better results will probably be obtained, since 
le small impurity of vapor may in the latter case be more accurately 
reated, like a gas. Moreover, care was taken to dry the bulbs at red 
eat prior to using them for air thermometry. Table 49 contains re- 
alts for bulbs thus dried, all unglazed internally, except No. 1, which 
5 the soldered form (Fig. 33), glazed internally. 
Table 49. — Moisture error of unglazed bulbs. 
No. 
H 
''o 
T 
Remarks. 
Porcelain : 
cm. 
cm. 
°C. 
No.l 
75.8 
94. 2 
99 
No.2. ...... 
75.7 
94.3 
99 
No.3 
62.6 
78.9 
102 
No.2 
35.7 
45.7 
104 
No.3 
48.5 
61.1 
102 
No.4 
76.3 
97.0 
102 
No. 4 
17. '26 
21.60 
101.1 
Cathetometer measurement. 
(853) 
