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y. Exhaust apparatus for ordinary pressure. (Comp. I PI. HE) 
This consists entirely of glass pieces of apparatus, to which the 
iron sucking-tube is connected by means of india-rubber. The gas’ 
from the gasholder is filtered through cotton wool and drawn through 
the tube Os Pl. III; if the cock O; is in the position shown here, 
the gas flows immediately to the pump along 029 and through the 
glasswool, phosphoric anhydride and fused sodiumhydrate in the 
tube Oo;. If we turn the cock 0; through 180° into its usual position, 
the gas passes through the washbottles 03, usually filled with pure 
sulphuric acid (comp. § 2 D. 2). In both cases the wide safety-tube 
Oy 18 connected, in which the mercury rises to barometric height during 
evacuation. The double washbottles are made so!) that they work 
in the same way with either direction of the gas-current. The 
bubbling of the gas through the sulphuric acid, which must remain 
perfectly clear, allows us, like the rising of the mercury in 0, to 
test very accurately whether the suction-valve works well and also 
if the pump-cylinder is properly exhausted. The pump-cylinder is filled 
with gas of less pressure than that in the apparatus from which it 
flows; we can reckon to take in from 100 to 125 liters of normal 
gas in an hour. 
Any spray from the sulphuric acid is received in the bulbs 03; 
and Oj; moreover the tubes are arranged so that the liquid flows 
back to the bottles; very fine drops that might be carried along, 
are retained in Oa. 
If the compression is stopped, the gas still left in the dome e4 
is caused to flow back through the cock #53 along Os, and O; to Og. 
The mercury-pump and accessories are exhausted through 04. The 
three-way-stopcock must then be so turned that the drying tubes 
communicate on both sides with the air-pump. For the exhaustion 
of the tubing of the apparatus (e.g. gas-holder) up to O; we can use Os. 
Il. To force compressed gas from a space of lower 
into one of higher pressure. 
As an instance of this we may consider the transference of gas 
from one cylinder to another (e.g. from an almost empty one into 
one not quite full, a case which often occurs when we wish to raise 
the pressure in the latter or to have the former at our disposal). With 
a view to this operation the chamber gg is made so that it can 
resist if necessary the full pressure of the pump. Fig. HI (PL III) 
1) Compare E. C, pe Vries. Thesis for the Doctorate. Leiden 1893. 
