KILN DRYING HANDBOOK. 
15 
relative humidity, and is what is meant when the term " humidity " 
is used in this bulletin. The lower relative humidities represent dry 
air and the higher ones moist air. Air at a temperature of 12'5° F. 
can hold a maximum of 40 grains of water vapor per cubic foot. If 
an atmosphere at that temperature had only 10 grains of water per 
cubic foot it would have only ten- fortieths of the maximum amount 
of water vapor it could hold, or a relative humidity of 25 per cent. 
Air with 25 per cent relative humidity is comparatively dry. The 
relative humidity of air having 30 grains would be thirty-fortieths, 
or 75 per cent, and would be considered moist. Air at 155° F. can 
hold 80 grains per cubic foot, or twice as much water vapor as at 
125°. At 155° air containing 10 grains of water per cubic foot would 
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Fig. 3. — Relation between the moisture in wood and the relative humidity in the sur- 
rounding atmosphere at three temperatures. Solid lines are based on actual data. 
be very dry, having a relative humidity of only 12^ per cent, and air 
containing 30 grains per cubic foot would still be moderately dry, 
having a relative humidity of 374 per cent. 
The amount of moisture in the air determines not only the rate 
but also the extent to which materials will dry. There is a definite 
balance between the humidity in the air and the moisture content 
of wood. All kinds of wood, if held long enough in an atmos- 
phere of constant temperature and humidity, will come to the same 
moisture content. The time required for this adjustment varies with 
different species. The relation between humidity in the air and 
moisture in the wood is an important one, since it forms the basis 
for some drying schedules and determines the extent to which wood 
for use under specified conditions of temperature and humidity 
should be dried. Figure 3 presents curves showing the humidity- 
moisture relation at three temperatures. The middle curve has been 
interpolated, actual data having been secured for the outer ones only. 
