BARUS.] 
PORCELAIN AIR THERMOMETRY. 
201) 
may be almost wholly excluded, the pressure 
chosen being, of course, that of the atmosphere. 
In this method, however, the volume of the bulb 
must be accurately known, a datum which is only 
of secondary importance in the constant-volume 
method. 
Again, the above data are obtained with spher- 
ical bulbs of the n on -re-entrant form. The diffi- 
culty experienced in obtaining a degree of satis- 
factory accordance in the various series of data 
is due to the fact that the environments are not 
identical. Hence in the following experiments the 
re-entrant form of bulb (Fig. 32) will be used, in 
which an identical exposure has been as nearly as 
possible realized. It will still be necessary to op- 
erate with bulbs not glazed internally. For final 
work bulbs of the re-entrant form, constructed in 
accordance with the Deville and Troost plan (Fig. 
33), so as to be easily glazed internally, are avail- 
able. 
A very convenient and simple apparatus for 
constant-pressure air thermometry is given in Fig. 
, yT)- actual size. The details of construction are 
very similar to those shown in Fig. 30, and it is 
therefore only necessary to indicate the essential 
points of difference. In the present instrument 
the platinum capillary tube A, the further end 
of which communicates with the air-thermometer 
bulb, is soldered with resinous cement into the 
top of a long cylindrical tube, B G. The length 
of this tube is at least 150 cm ; it is accurately grad- 
uated in cubic centimeters, and the total capacity 
is about 300 cc . In my apparatus the tube B G was 
closed below with a rubber cork, and this end 
then inserted with plaster of Paris into a suitable 
rest or foot, By removing the foot and the cork 
the tube admits of being cleaned, an operation 
which, for the case of imperfectly pure mercury, 
s sometimes necessary. Practically the tube is 
losed below, but it is provided at a short dis- 
;ance above its end with a horizontal tubulure, 
t) E, to which a cloth-wrapped rubber hose, E F G, 
s attached. The upper end of E F G communi- 
cates by means of a three-way cock, H, with a large 
;yliudrical vessel of mercury, K L L. The cock 
U has a lateral tubulure, 
Bull, o4— 14 
h, through which mer- 
