the stop-cocks C and C_1 being open, and all other connections closed,
the reservoir R_2 was raised so far that the mercury filled the
reservoir R_1 and a part of the narrow connecting U-shaped tube. When
the pump was set to work, the mercury would, of course, quickly rise
in the tube, and reservoir R_2 was lowered, the experimenter keeping
the mercury at about the same level. The reservoir R_2 was balanced
by a long spring which facilitated the operation, and the friction of
the parts was generally sufficient to keep it almost in any position.
When the Sprengel pump had done its work, the reservoir R_2 was
further lowered and the mercury descended in R_1 and filled R_2,
whereupon stopcock C_2 was closed. The air adhering to the walls of
R_1 and that absorbed by the mercury was carried off, and to free the
mercury of all air the reservoir R_2 was for a long time worked up and
down. During this process some air, which would gather below stopcock
C_2, was expelled from R_2 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 R_2 when it was lowered, the caustic potash was resorted to.
The reservoir R_2 was now again raised until the mercury in R_1 stood
above stopcock C_1. 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
R_2 was for a long time raised and lowered. In this manner all the
moisture was carried off from the mercury, and both the reservoirs
were in proper condition to be used. The reservoir R_2 was then again
raised to the top, and the pump was kept working for a long time. 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 R_2 was lowered, and upon reservoir R_1 being emptied the
receiver r was quickly sealed up.
When a new bulb was put on, the mercury was always raised above
stopcock C_1 which was closed, so as to always keep the mercury and
both the reservoirs in fine condition, and the mercury was never
withdrawn from R_1 except when the pump had reached the highest degree
of exhaustion. It is necessary to observe this rule if it is desired
to use the apparatus to advantage.
82