CHAPTER II. 
THE CALIBRATION OF ELECTRICAL PYROMETERS BY THE AID 
OF FIXED THERMAL DATA. 
EXPLANATION. 
Iii the apparatus described in the foregoing chapter, great masses of 
metal were kept at the boiling point. The advantages gained from a 
brisk but perfectly free circulation of vapor, particularly in the case 
where the vapor is of small specific heat, have already been pointed 
out. It is to the use of large apparatus that Messrs. Deville and Troost 
had been led before us. In their experiments, however, the direct use 
of the air-thermometer made a boiler of considerable size essential from 
the outset. 
Large and expensive apparatus, whatever be their special advantages, 
can never enjoy extensive use in the general laboratory. It is there- 
fore the object of the present chapter to describe special forms of appa- 
ratus by which calibrations of tiiermo-elements may be quickly and safely 
made, and by which the problem of thermo-electric temperature measure- 
ment may be reduced to an ordinary laboratory experiment. 1 The de- 
gree of error to which the observer is liable, the degree of constant tem- 
perature attained, the selection of substances having convenient boiling 
points, and finally the application of the apparatus to a variety of sub- 
stances for boiling-point measurement will constitute the chief topics of 
this chapter. The boiling-point apparatus must of course be such that 
ebullition may be kept up indefinitely. 
APPARATUS FOR LOW BOILING POINTS (100° TO 500°). 
Original forms of boiling-point tube. — The original forms of boiling- 
point apparatus for mercury, for sulphur, and for aniline, water, etc., 
are given in Figs. 7, 8, 9, drawn to a scale of £. They are all constructed 
on essentially the same principle, slight modifications being introduced 
to meet each case. The apparatus, Fig. 7, consists of an ordinary glass 
lamp chimney, aaaa, inverted as shown, and closed at its lower end by 
a plaster of Paris ping, bb. surrounded by a wrought-iron cap, cc. The: 
cap cc is larger than the glass, and by pouring in the plaster in the 
moist condition and allowing it to set, the tube is firmly secured between 
1 Since the completion of the work of the present chapter, M. Le Chatelier has made 
pyrometric experiments with ends in view similar to those here proposed. (Cf. p. 50.)| 
«4 (738) 
