21 
THE RARER ELEMENTS IN SOILS AND PLANTS. 
The essential part of the automatic control is a couple consisting 
of a quartz tube closed at one end, inside of which is placed a nickel 
rod. The difference in the thermal expansion of nickel and silica is 
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ASSERT 
By this device a movement of 0.01 inch for every 24° 
C. was obtained. No change in temperature of the oven running 
at 525° C. could by observed with a pyrometer graduated in 5-degree 
intervals. 
considerable. 
