BAKU8.J DEGREE OF CONSTANT TEMPERATURE. 67 
alternately to the two furnaces at regular intervals (ten minutes) a com- 
paratively constant condition could be maintained until the accumula- 
tion of ashes interfered with the draft. The furnace performed its 
part of the work satisfactorily, but from the large dimensions at least 
one day was necessary to repair damages and prepare for a new "heat," 
and sometimes several days were necessary to get the crucible cleaned 
and ready again. 
The crucible was filled with pieces of zinc and the powder and grains 
from previous distillings, mixed with powdered charcoal as a reducing 
agent. The cover was luted on as stated, and the tank placed under 
the stone pipe N N. In operation the vapor from the boiling metal 
rose around the sphere K and passed through holes into it, and down 
through Or into 1ST N, where it condensed and fell into the water, thus 
keeping the pipe L and the thermo-element contained therein at a 
comparatively constant temperature. The degree of constancy actually 
attained will be fully discussed later. Even iu this apparatus the burn- 
ing of the vapor proved a source of endless and unavoidable annoyance. 
There seems little doubt that the vapor of zinc is even able to decom- 
pose water vapor and liberate the hydrogen, itself producing a horn-like 
oxide, which is quite as apt to clog the outlets as the solid metal. Ow- 
ing to this, many and frequent were the cases where the u heat" proved 
incomplete, or a total failure, owing to stoppages and explosions or 
leaks. This will account for the incomplete series of determinations 
which may occur in subsequent tables. Before we were able thoroughly 
to profit by our experience the transfer of the laboratory to Washing- 
ton interrupted the work and gave it a different direction. 
EXPERIMENTAL RESULTS. 
Methods of measurement. — Whether two given temperatures are equal 
or not may be shosvn with great accuracy and certainty by thermo-elec- 
tric comparison. Thermocouples of platinum with palladium, low 
percentage alloys of platinum-iridium, platinum-nickel, platinum-rho- 
dium, platinum-cobalt, and many others, are available for the purpose. 
Furthermore, since comparatively small increments of temperature are 
here to be observed, the degree of constant temperature obtained in 
any given space both as regards its variation at any given point with 
time, as well as the distribution of temperature existing in the said 
space at a given time, can be fairly estimated by thermo-elements of 
known power. 
Many observations go to show that for practical purposes we may rep- 
resent the partial electro-motive force at each junction of a thermo-ele- 
ment by an equation of the form 
e T =aT+bT 2 +eT'+ (1) 
where e T is the (partial) electromotive force at the junction whose tem- 
perature is T, and a,b, c, . . . a/e thermo-electric constants rapidly 
decreasing in magnitude. 
(721) 
