reservoir R_1. Especial care was taken in fitting the grinding
surfaces of the stoppers p and p_1, and both of these and the mercury
caps above them were made exceptionally long. After the U-shaped tube
was fitted and put in place, it was heated, so as to soften and take
off the strain resulting from imperfect fitting. The U-shaped tube was
provided with a stopcock C, and two ground connections g and g_1--one
for a small bulb b, usually containing caustic potash, and the other
for the receiver r, to be exhausted.
The reservoir R_1 was connected by means of a rubber tube to a
slightly larger reservoir R_2, each of the two reservoirs being
provided with a stopcock C_1 and C_2, respectively. The reservoir R_2
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 C_2 closed, so as to form a Torricellian vacuum in it when
raised, it could be lifted so high that the mercury in reservoir R_1
would stand a little above stopcock C_1; and when this stopcock was
closed and the reservoir R_2 descended, so as to form a Torricellian
vacuum in reservoir R_1, it could be lowered so far as to completely
empty the latter, the mercury filling the reservoir R_2 up to a little
above stopcock C_2.
The capacity of the pump and of the connections was taken as small as
possible relatively to the volume of reservoir R_1, since, of course,
the degree of exhaustion depended upon the ratio of these quantities.
With this apparatus I combined the usual means indicated by former
experiments for the production of very high vacua. In most of the
experiments it was convenient to use caustic potash. I may venture to
say, in regard to its use, that much time is saved and a more perfect
action of the pump insured by fusing and boiling the potash as soon
as, or even before, the pump settles down. If this course is not
followed the sticks, as ordinarily employed, may give moisture off at
a certain very slow rate, and the pump may work for many hours without
reaching a very high vacuum. The potash was heated either by a spirit
lamp or by passing a discharge through it, or by passing a current
through a wire contained in it. The advantage in the latter case was
that the heating could be more rapidly repeated.
Generally the process of exhaustion was the following:--At the start,
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