barus.1 PORCELAIN AIR THERMOMETRY. 2P>7 
/3 were known from special measurements this value could be used to 
compute 2. It is obvious therefore that for sharp values of ft it is 
necessary to work with a jacketed manometer, so that T x may be con- 
stant. It is necessary also that T and R be constant, a condition which 
the above experiments do not quite fulfill. The above data, crude as 
they are, show, however, that /?is determinable by this method with the 
same degree of accuracy with which it is to be applied. In this respect 
the constant-pressure method is unique, since it admits of easy modifications 
by which the zero volume of the bulb, its coefficient of expansion, as well as 
allpermanent changes of volume, may be evaluated ivithout extra appliances. 
[Attention may again be directed to the independent method of standard- 
ization of a non-inglazed re-entrant porcelain air-thermometer bulb by 
thermal comparison with a re-entrant glass thermometer bulb of known 
constants. Such comparison is to be made above 300° to obviate the 
moisture and condensation errors, and either directly in the elliptic re- 
volving muffle (Fig. 36 a), or indirectly through the intervention of the 
same thermo-couple. In the last case each bulb is separately compared 
with the couple as explained above (Figs. 37, 38), and the results then 
co-ordinated. The hard-glass bulb, according to Troost (loc. cit.) may 
be safely regarded rigid as far as' 600°. 1880.] 
Remarks. — The manner of further development of the present ther- 
mal problems is now sufficiently obvious, and may be briefly summar- 
ized in a final remark. It is necessary in the first place to rigorously 
compare bulbs glazed interiorly with bulbs not glazed interiorly. The 
latter are so much more easily constructed that if their use be warranted 
practical air thermometry will be in no small measure facilitated. For 
instance, if we suppose the bulb non-glazed interiorly to be admissible, 
then there are no serious obstacles in the way of a fire-clay thermometer 
bulb. Bulbs of such ware are naturally porous, but there is no doubt 
that enamel can be applied in sufficient quantity to the exterior to make 
them impervious to air. With these bulbs the upper limit of possible 
thermal measurement will closely approach the melting point of pla- 
tinum. By aid of the volumetric method described on pages 195 and 
214, problems referring to the internal volume of the bulbs and stems, 
whether porous or not, admit of satisfactory solution. Again, it is nec- 
essary to compare the data of bulbs containing different gases, dry air, 
2 , H 2 , N 2 , etc. All such comparisons can be made either directly, by 
exposing the bulbs contiguously in an elliptic revolving muffle of the 
kind sketched and described on pp. 182, 188, or they may be made indi- 
rectly, by comparing the individual air-thermometer bulbs with the same 
thermo-couple. In the interest of greater accuracy the same re-entrant 
form of bulb, into which the divers gases are successively introduced, 
is expediently combined with one and the same thermo-couple, and the 
heating is conducted precisely in the manner shown in this chapter. 
Until Charles's law has in this way been tested for large ranges of tem- 
perature it is hardly desirable to multiply the number of approximate 
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