278 
MR. C. T. R. WILSON ON CONDENSATION OF WATER VAPOUR 
from contamination with air, and only comes in contact with glass and mercury. It 
has been obtained free from dissolved gases by boiling distilled water rapidly down 
to about one-sixth of its bulk in the tlask shown on the right. While it is boiling 
down, R] is repeatedly raised and lowered, to wash out any imprisoned air or 
unboiled water from B and the tube leading to the flask. 
While the water is still boiling, B is filled by lowering Ptj, a depth of about 
1 centim. of mercury being, however, always left in B to prevent the water from 
coming in contact with the indiarubber stopper which closes the lower end of the 
tube. The tap To is then closed and Pt fixed high enough to give the requisite 
pressure. 
Introduction of the Gas. 
The apparatus used in the preparation of the various gases was, in all cases, made 
entirely of glass, all joints being made with the blow-pipe. It was fused on to the 
end of the tube H (fig. 2). The methods of preparing the various gases are described 
later. The whole gas generating apparatus must first be filled with the pure gas up 
to the tap To, which is now kept closed. 
Before introducing the gas into the expansion apparatus, A and B must first be 
filled with air-free water. This is done in the following way. 
The piston P is drawn up to the top of A by opening the tap and lowering the 
mercury reservoir Bj, the other taps being closed. The gas which has collected in B 
is then driven out through Tg by raising II^ after closing Water is then drawn 
from the flask F, which is kept boiling the whole time, into B, and Tg is closed. 
Again, Tg is opened and water is introduced into A, driving the piston before it. A 
now contains water with a bubble of air at the top ; this is driven into B by lowering 
Pg. The tap Tg is closed, while the greater part of the water still remains in A, from 
which it slowly escapes by the floating up of the piston. 
The small quantity of water which remains above the piston has, of course, been 
contaminated by contact with the air or other gas which originally occupied A. To 
replace it by pure water, B is first filled with air-free water as described in the last 
section. A small quantity of this is then passed into A while still hot, so that it 
floats above the cold water already there. It is, therefore, mainly the latter which 
flows away as the piston gradually rises. More water is run in from B before the 
piston quite reaches the top of A. This process is repeated two or three times, and 
finally the piston is driven riglit dovvn to the bottom of A, which is thus completely 
filled with water. The tap Tg is left open and Ilg raised to a considerable height, 
and the piston thus pressed down into the constriction so that the water does 
not escape. 
When the apparatus has cooled, the piston is drawn up by lowering Bg, but is not 
