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Updated: May 3, 2025
During this process some air, which would gather below stopcock C2, was expelled from R2 by lowering it far enough and opening the stopcock, closing the latter again before raising the reservoir. When all the air had been expelled from the mercury, and no air would gather in R2 when it was lowered, the caustic potash was resorted to.
Generally the process of exhaustion was the following: At the start, the stop-cocks C and C1 being open, and all other connections closed, the reservoir R2 was raised so far that the mercury filled the reservoir R1 and a part of the narrow connecting U-shaped tube.
When the highest vacuum obtainable with the pump had been reached the potash bulb was usually wrapped with cotton which was sprinkled with ether so as to keep the potash at a very low temperature, then the reservoir R2 was lowered, and upon reservoir R1 being emptied the receiver r was quickly sealed up.
The reservoir R2 could be raised and lowered by a wheel and rack, and the range of its motion was so determined that when it was filled with mercury and the stopcock C2 closed, so as to form a Torricellian vacuum in it when raised, it could be lifted so high that the mercury in reservoir R1 would stand a little above stopcock C1; and when this stopcock was closed and the reservoir R2 descended, so as to form a Torricellian vacuum in reservoir R1, it could be lowered so far as to completely empty the latter, the mercury filling the reservoir R2 up to a little above stopcock C2.
The reservoir R2 was now again raised until the mercury in R1 stood above stopcock C1. The caustic potash was fused and boiled, and the moisture partly carried off by the pump and partly re-absorbed; and this process of heating and cooling was repeated many times, and each time, upon the moisture being absorbed or carried off, the reservoir R2 was for a long time raised and lowered.
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