NOTES AND PROBLEMS 43The piston, which is assumed to be absolutely gas tight, is raised
to a middle position in the cylinder and fastened there. The
stop cock to the vacuum pump is opened and the cylinder evacu-
ated until the pressure gauge reads zero. Now the cock to the
pump is closed and the cock from the water reservoir is opened to
let a thin layer of water run into the bottom of the cylinder.
Immediately the pressure gauge jumps and shortly adjusts itself
to exactly 17.4 mm. when the temperature of the apparatus
is 20°C.
Now let us raise the piston to the top of the cylinder. There
is a momentary depression of the gauge but it at once returns to
exactly 17.4 mm. Now let us force down the piston to the bot-
tom of the cylinder. Again there is a momentary change in the
gauge level but the reading immediately becomes exactly 17.4
mm. again, if the temperature is maintained constant all the time
at 20°.
Saturated water vapor is in equilibrium with liquid water. If
the vapor is not saturated and any liquid water is present, enough
of this evaporates to make the vapor saturated. Compressing
a saturated vapor would tend to increase its concentration, which
would increase its pressure. The observed fact that there is no
such increase in pressure shows that the vapor does not become
more concentrated, that is, that it does not become supersaturated
but condenses to liquid water to maintain the exact state of satura-
tion as the piston is pressed down. When the piston is drawn up
the liquid must evaporate to maintain a saturated condition of the
vapor.
At 20° the concentration of saturated water vapor is such that
its pressure is 17.4 mm. At other temperatures the concentration
is different, but it has an absolutely definite value for each tem-
perature. The pressure of saturated water vapor has been care-
fully measured at different temperatures. The values from 0°
to 100° are given in the table on page 353 in the Appendix.
If the condition of equilibrium between liquid water and water
vapor is not disturbed by the presence of any other gas, then a
cylinder full of oxygen and also saturated with water vapor will
contain the same quantity of water vapor as a cylinder of the
same size containing only saturated water vapor. Dalton's law
would then require that the partial pressure of water vapor be
the same in both cylinders and that the apparent pressure in the