406 ME. C. T. E. WILSON ON THE CONDENSATION NUCLEI PEODUCED IN 
to the light glass plunger, P, Vv-liich slides freely over it. The plunger is made from 
a thin-walled test-tube, the open end of which has been cut perpendicular to the 
sides and ground smooth. Its lower edge is always immersed in the mercury which 
fills tlie lower part of B, and thus the gas in A and the upper part of B is completely 
cut olf from the air inside P. The external diameter of the plunger is 2 miUims. 
less than the internal diameter of the outer tube; there is thus a space of 1 millim. 
all round the tubes. When the tap, Tj, is open and there is thus free communication 
between the space inside P and the atmosphere, the plunger rises till the j)ressure 
in A only differs from the atmospheric pressure by an almost negligible amount, 
depending on the difterence between the weight of the 23lunger a,nd of the mercury 
displaced by the immersed part of its walls. If, now, communication with the 
atmosphere be cut off (by closing the tap Tj), and the space below the plunger be 
suddenly connected with the vacuum in F by means of the valve, V, the plunger is 
driven through the mercury till it strikes the indiarubber, against which it remains 
tightly held by the pressure of the air above it. The mercury remains practically 
stationary, while the thin edge of the plunger cuts its way through it. 
If Ti be again opened, re-admitting air into the space below the j^lunger, the latter 
rises to its original position, and an expansion of the same amount can be rej^eated 
as often as may be recpnred. To arrange for an expansion of any given amount, the 
tap, Ta, must be opened Avhile the plunger is in contact with the indiarubber, that is, 
in the position it occupies immediately after an expansion. The mercury reservoir, 
R, is then fixed at such a level that the pressure in A, as indicated by the gauge, is 
the desired amount beloAv that of the atmos})here; the tap, T 2 , is then closed and the 
plunger made to rise by opening the tap, Tj. 
If B be the barometric pressure, then the pressure of the gas before expansion is 
Pi = B d- — 77, 
where 77 is the vapour pressure at the temperature of experiment, and m is the pressure 
(amounting to 1 or 2 millims. of mercury) required to keep the Avails of the plunger 
immersed in the mercury [m is measured by finding the pressure which has to be 
applied to the air in A to keep the piston immersed to the same depth when the space 
below it is in communication Avith the atmosphere). 
The pressure of the gas after expansion is 
P 2 = B — p — tt, 
v/here p is the difference of pressure indicated by the open mercury gauge Avhen put 
in connection Avitli A before the previous contraction. 
Then the ratio of the final to the initial volume of the gas is (if Boyle’s laAV holds) 
V3 P, E -f- m — 77 
P., P) — p — IT 
