THOMPSON. — ON THE EQUILIBRIUM OF THE SYSTEM. 



433 



of the lid containing a mica window, making it possible to see the hot 

 material held in the center of the spiral. The support for the crucible 

 is a graphite rod held by the lower electrode, but insulated by lava 

 rings. The lid also contains a pipe by which the furnace may be ex- 

 hausted. A Geryk oil pump was used for obtaining the vacuum. The 

 pressure could be read by a wooden scale divided in millimeters on a 

 mercury gauge completely evacuated and sealed off at one end, thus 



400 



600 



800 1000 1200 



Temperature Indicated 



1400 



1600 



Figure 1. Calibration of Thermoelectric Junction. 



making a siphon barometer. The temperature of the gas contained in 

 the furnace is not constant, but all that determines the equilibrium 

 besides the pressure is the temperature of the solid substances and of 

 the gas in contact with it. Of course the pressure must be constant 

 throughout the furnace. 



In the first experiments the temperature was measured by a Wanner 

 pyrometer, which rendered it necessary to replace the mica window by 

 one of glass clamped between rubber and sealed up with paraffin. In 

 calibrating the pyrometer a similar piece of glass was placed between 

 the amylacetate standard and the instrument. The Wanner was found 



VOL. XLV. — 28 



