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©. 1. We must always expect the possibility of the mercury 
returning, or what sometimes could be worse, of the already com- 
pressed gas returning into the apparatus, from which it is taken, 
in consequence of leakage along forcing and suction valves, espe- 
cially when the working of the pump is interrupted for some time 
for one reason or other. This returning of the mercury might cause 
great disturbances and even accidents. In CaILLeret’s original 
pump a feeding valve (« Pl. I fig. 1) has sometimes been used but 
afterwards this has been again removed. When this small ebonite 
valve came into use, and was closed Ey a pressure of for instance 100 
atmospheres, it thereby stuck and the pump was stopped working. 
It could not then be started again without very complicated opera- 
tions, if we did not wish to lose gas or to have it contaminated 
with air. Moreover this valve was an obstruction to the easy entrance 
of the gas into the cylinder. 
In the newly constructed compressor a safety-feeding-valve (comp. 
gio Pl. HI) has been constructed which in ordinary circumstances 
lies loosely under its support but is raised and pressed against this 
by the mereury, as soon as this forces its way towards the exhaust 
side, while the valve can be loosened from the support from the 
outside, after having been pressed against it by high pressure, with- 
out opening the pump. 
E 2. At the same time we can avail ourselves of this valve 
for closing the pump at the exhaust-side, and this is always done as 
soon as we stop the working of the pump. 
When started again after an interruption or stoppage with the 
valve closed, the pump begins to exhaust the sucking-chamber as 
far as the valve, and then by opening this we can again make the 
connection with the exhaust-tube. 
€. 3. As with the least contamination of the mercury (and 
especially when small splinters of iron or other particles of hard 
material are in the pump) the valves cease to be perfectly closed, 
part of the mercury may flow over to the exhaust-side. In order 
to prevent this from getting into the apparatus, from which the 
gas is taken, an antechamber gg (PL. III) is made, which if neces- 
sary can contain the whole quantity of mercury which is above 
the exhaust-valve in the pump cylinder. 
%. The compressed gas must be entirely freed from mercury. 
The spreading of the gas jet in the dome shaped reservoir, in which 
(in CAILLETET’s pump) it is compressed before it is admitted into 
the outflow pipe is partly useful to this end. 
