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8. 1. It appeared however an advantage to add a spray catcher 
and separating plate. 
8. 2. Further separation is promoted by passage through a small 
cylinder %, (Pl. III) in which the current of gas is once more 
forced to change its direction, while 
8 3. the last traces of mercury are removed by a mercury-filter 
ks (Pl. ITT) in which the mercury is brought into contact with 
copper and gold-leaf. 
©. In order to be warned when the mercury passes over into 
either the chamber gs or into the overflowvessel £3, and to observe 
the position of the mercury in the compression tube C’, insulated 
contacts are taken through the steel walls, of which those in g, and 
kg are permanently connected with an electric bell, while that in C 
only makes contact through a control switch. 
§. The above indicates what is necessary for sucking in the 
pure gas at the exhaust-side and for forcing it out at the pressure- 
side under high pressure and free from mercury. 
In order to be able to work regularly with the pump we must 
still contrive some additional apparatus. Among these are: 
DH. 1. A number of cocks, several of which are united together 
on a cock-board, which also gives 
§. 2. an opportunity for measuring not only the tension in the 
reservoir of the pump (as in CAILLETET's pump), but also that in 
the apparatus in which the gas is being compressed. 
§. 3. A safety-valve, which bursts whenever the pressure be- 
comes high enough to endanger one of the pieces of apparatus which 
are under pressure and joined on to the pump. 
5. 4. a safety-tube on the exhaust-side, for protecting the appa- 
ratus to be connected with it, ; 
DH. 5. a connection with the air-pump so that the compressor and 
all the accessories can be exhausted before pure gas is admitted 
into them. 
The operations which are made with the pump, may be arranged 
under three different heads, which we can find for ourselves from 
the diagram if we connect I, II and III with the exhaust-tube of 
the pump. But before treating of these three operations we will 
first describe the pump itself more in detail. 
§ 3. a. Packing of the compression-tube (see Pl. IV). The 
piston-rod or plunger 4’; moves in a lignum vitae coating b3 fitting 
in a cavity turned in the compression-tube C'. Before working the 
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