20 BULLETIN 600, U. S. DEPARTMENT OF AGRICULTURE. 
omitting sulphuric acid during the ignition. In this laboratory, 
where the voltage of the electric current varies considerably, the very 
frequent attention of the analyst is required to operate the rheostat 
of the electric furnace in order to keep the temperature approxi- 
mately uniform. The difference in radiation from the furnace on 
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Fie. 1.—Diagram of electric furnace, side view. 
hot and cold days is another factor which causes variation in tem- 
perature when the rheostat is set at the same place. To secure a 
fairly constant temperature without the element of guesswork, a 
furnace with a temperature regulator was designed. A sketch is 
given in figs. 1, 2, and 3. 
