HUMIDITY-REGULATED AND RECIRCULATING DRY KILE". 



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 pi^essures 

 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, yet 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 



