HUMIDITY-REGULATED AND RECIRCULATING DRY KILN. 3 
there is no air leakage. The qualification in regard to air is neces- 
sary, for the sake of exactness, for the following reason : In any given 
space the total actual pressure is made up of the combined pressures 
of all the gases present. If the total pressure ("vacuum") is 2 
inches, and there is no air present, it is all produced by the water 
vapor (which saturates the space at 101° F.) ; but if some air is pres- 
ent and the total pressure is still maintained at 2 inches, then there 
must be less vapor present, since the air is producing part of the 
pressure and the space is no longer saturated at the given tempera- 
ture. Consequently further evaporation may occur, with a cor- 
responding lowering of the temperature of the water, until a balance 
is again reached. Without further explanation it is easy to see that 
but little water can be evaporated by a vacuum alone without addi- 
tion of heat and that the prevalent idea that a vacuum can of itself 
produce evaporation is a fallacy. If heat be supplied to the water, 
however, either by conduction or radiation, evaporation will take 
place in direct proportion to the amount of heat supplied, so long as 
the pressure is kept down by the pump. 
At 30 inches of mercury pressure (one atmosphere) the space be- 
comes saturated with vapor and equilibrium is established at 212° F. 
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 what 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 equlibrium between 
temperature and vapor pressure is resumed. 
Relative humidity is simply the ratio of the actual vapor pres- 
sure 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° F. the space is saturated, and since the pressure of satu- 
rated 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, j^et the space would still be saturated at the given tempera- 
ture. Again, if the temperature were 101° F., the pressure of satu- 
rated vapor would be only 1 pound, and the additional pressure of 
