CHAPTEE I. 
THE DEGREE OF CONSTANT HIGH TEMPERATURE ATTAINED IN 
METALLIC VAPOR BATHS OF LARGE DIMENSIONS. 
By C. Barus and W. Hallock. 
EXPLANATION. 
The general character of this chapter is introductory. The experi- 
ments are therefore largely arranged with reference to a single point of 
view, viz, the degree of constancy attainable in metallic vapor baths 
at temperatures either high or low. To secure this end we profited by 
the experience of Messrs. Deville and Troost, who recommend the use of 
large masses of metal, and consequently large forms of boiling-point 
apparatus. The advantages gained in this way are twofold. It is ob- 
vious that if the quantity of substance used is indefinitely large, not 
only may the ebullition be prolonged for a considerable time — a great 
desideratum when many pyrometers are to be calibrated — but in the 
same measure, as the volume of vapor and consequently the space of 
constant temperature is larger, the probability of constant temperature 
at any one central point is proportionately increased. 
There is, however, a second point of view from which the present ex- 
periments have been pursued. The questions to which this leads us 
do not conflict with the main purpose of the chapter. They are inti- 
mately connected with the thermal equivalents of the thermo-electric 
indications of small temperature variations upon which many of the 
following data must be based. The refractory alloys best adapted for 
measurements of high temperature are invariably of platinum body, in 
which relatively small amounts of the foreign metallic ingredient, are 
added to platinum. This will be seen more fully below. Now it ap- 
pears that alloys containing platinum and the other metal mixed in 
nearly equal amounts are either brittle or otherwise unsuited for wires. 
Often the second ingredient is of a relatively volatile or oxidizable 
kind. Escaping at high temperatures, this gives rise to changes of 
homogeneity or even of composition. Fortunately it is not difficult to 
find low percentage platinum alloys, which, when combined with pure 
platinum thermo-electrically, show electromotive forces so large that no 
difficulties of measurement can be apprehended. In fact the purely 
electrical measurement is a part of the problem which may be solved 
with extreme nicety. 
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