wn by the vacuum pump.

At 30 inches of mercury pressure (one atmosphere) the space becomes
saturated with vapor and equilibrium is established at 212 degrees
Fahrenheit. If heat be now supplied to the water, however, evaporation
will take place in proportion to the amount of heat supplied, so long
as the pressure remains that of one atmosphere, just as in the case of
the vacuum. Evaporation in this condition, where the vapor pressure at
the temperature of the water is equal to the gas pressure on the
water, is commonly called "boiling," and the saturated vapor entirely
displaces the air under continuous operation. Whenever the space is
not saturated with vapor, whether air is present or not, evaporation
will take place, by boiling if no air be present or by diffusion under
the presence of air, until an equilibrium between temperature and
vapor pressure is resumed.

Relative humidity is simply the ratio of the actual vapor pressure
present in a given space to the vapor pressure when the space is
saturated with vapor at the given temperature. It matters not whether
air be present or not. One hundred per cent humidity means that the
space contains all the vapor which it can hold at the given
temperature--it is saturated. Thus at 100 per cent humidity and 212
degrees Fahrenheit the space is saturated, and since the pressure of
saturated vapor at this temperature is one atmosphere, no air can be
present under these conditions. If, however, the total pressure at
this temperature were 20 pounds (5 pounds gauge), then it would mean
that there was 5 pounds air pressure present in addition to the vapor,
yet the space would still be saturated at the given temperature.
Again, if the temperature were 101 degrees Fahrenheit, the pressure of
saturated vapor would be only 1 pound, and the additional pressure of
14 pounds, if the total pressure were atmospheric, would be made up of
air. In order to have no air present and the space still saturated at
101 degrees Fahrenheit, the total pressure must be reduced to 1 pound
by a vacuum pump. Fifty per cent relative humidity, therefore,
signifies that only half the amount of vapor required to saturate the
space at the given temperature is present. Thus at 212 degrees
Fahrenheit temperature the vapor pressure would only be 7-1/2 pounds
(vacuum of 15 inches gauge). If the total pressure were atmospheric,
then the additional 7-1/2 pounds would be simply air.

"Live steam" is simply water-sat