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gases. A portion of the boiled liquid is then poured into the test-tube so 
as to fill it two-thirds full. By partially closing the screw-clip, vapour from 
the boiling solvent is caused to “blow through” the liquid in the test-tube. 
When all air has been expelled, the stopper is inserted and the screw-clip 
simultaneously opened. After temperature equilibrium has become estab- 
lished, the vapour pressure of the solvent within the test-tube is compared 
with the barometric pressure by observing the difference of level between 
the liquid in the test-tube and the gauge tube. If the solvent is pure, these 
two pressures are identical ; in any case, the blowing-through process is 
repeated till constant results are obtained. 
A known weight of solute is now added, the air expelled, the stopper 
inserted, and the reading of difference of level made, just as before, after 
