6 BULLETIN 509, U. S. DEPARTMENT OE AGRICULTURE. 
space, whether any air is present or not; and the pressure of the 
vapor is the same in both cases. The total pressure (as registered by 
the gauge) will not be the same, however, since if air is present its 
pressure is added to that of the vapor. It is really the space and 
not the air which is saturated. For instance, at 101° F. it takes about 
20 grains of vapor to saturate a cubic foot of space. If no air be 
present, there will be a pressure of vapor only, which will be about 
1 pound, or a vacuum of 28 inches. If this is open to the atmosphere 
the air will rush into the space until the total pressure will be one 
atmosphere, or about 15 pounds. There will then be 1 pound of pres- 
sure produced by the vapor, as before, and 14 pounds of air pressure. 
The space will still be saturated, if the temperature is kept at 101° F. 
If this is now heated to 160° F. and open to the atmosphere so that 
the total pressure is kept constant, the ratio of the pressures of vapor 
and air will also remain the same ; there will still be 1 pound due to 
vapor and 14 pounds due to the air. (The weights in the cubic foot 
of space of both will decrease, due to expansion by heat.) At 160° F., 
however, it requires 91 grains of vapor to saturate a cubic foot of 
space, and its pressure is nearly 5 pounds (absolute). Consequently, 
the relative humidity at 160° F. of this space will be one-fifth, or 20 
per cent. Conversely, if this air and vapor at 20 per cent relative 
humidity and 160° F. temperature is cooled to 101° F., all at the same 
atmospheric pressure, the space will again become saturated, and any 
further cooling will cause precipitation or condensation. This is 
called the dew point; that is, 101° F. is the dew point of air with 20 
per cent humidity at 160° F. In Forest Service Bulletin 101, " Prin- 
ciples of Drying Lumber at Atmospheric Pressure and Humidity 
Diagram." a humidity diagram is given for solving all problems of 
this nature. The concave curves on this diagram are simply curves 
of constant vapor pressure with change of temperature and relative 
humidity, and the grains of vapor per cubic foot, at saturation or the 
dew point, are given in numerical figures. From this it is seen that 
the dew point determines the relative humidity when the temperature 
is raised, or vice versa. If we take saturated air at known tempera- 
ture and heat it up any given desired amount, the resulting relative 
humidity is thereby determined. This is the principle upon which 
the humidity regulation depends in a new kiln designed by the 
writer. 1 It is also evident that whenever air is cooled below its dew 
point condensation takes place. This is the principle of the con- 
denser. There are a number of kilns which have made use of this 
principle to dry the air. Pipes are used for the condensers and cold 
water is circulated through the pipes. The same thing can be accom- 
plished by a spray of cold water in place of the pipes, provided all 
the fine mist is subsequently removed from the air, or even by a sur- 
1 For a description of this kiln see page 10. 
