2 BULLETIN 509, U. S. DEPARTMENT OF AGRICULTURE. 
well as very small ones, irregularly arranged. Consequently, strains 
are produced when the wood dries, which cause warping and check- 
ing. While air drying is undoubtedly the safest method, the process 
is ordinarily so slow, requiring a year or longer according to species 
and size, that forced " artificial " drying becomes a business neces- 
sity. Moreover, air drying is by no means always to be preferred to 
kiln drying from the standpoint of the quality of the product. 
A correct understanding of the principles of drying is rare, and 
opinions in regard to the subject are very diverse. The same lack of 
knowledge exists in regard to dry kilns. The physical properties 
of the wood which complicate the drying operation and render it 
distinct from that of merely evaporating free water from some sub- 
stance like a piece of cloth must be studied experimentally. It can 
not well be worked out theoretically. 
The thermal process of the drying operation, however, is capable 
of exact theoretical analysis. It is the purpose of this article to 
interpret the conditions which exist in the various stages of the dry- 
ing operation with respect to the heat quantities and the changes 
which occur in the drying medium, from a theoretical standpoint. 
The object of this analysis is to show the limiting conditions which 
may be approached, but can not be exceeded. 
ELEMENTARY PRINCIPLES OF DRYING. 
Before taking up the theoretical discussion, a few remarks upon the 
elementary principles of drying will be of assistance. 
EVAPORATION REQUIRES HEAT. 
In the first place, it should be borne in mind that it is the heat 
which produces evaporation and not the air nor any mysterious 
property assigned to a " vacuum.'' For every pound of water evapo- 
rated at ordinary temperatures approximately 1,000 British thermal 
units of heat are used up, or " become latent," as it is called. This 
is true whether the evaporation takes place in a vacuum or under a 
moderate air pressure. If this heat is not supplied from an outside 
source it must be supplied by the water itself (or the body being 
dried), the temperature of which will consequently fall until the sur- 
rounding space becomes saturated with vapor at a pressure cor- 
responding to the temperature which the water has reached ; evapora- 
tion will then cease. The pressure of the vapor in a space saturated 
with water vapor increases rapidly with increase of temperature. 
At a so-called vacuum of 28 inches, which is about the limit in com- 
mercial operations, and in reality signifies an actual pressure of 2 
inches of mercury column, the space will be saturated with vapor 
at about 101° F. Consequently, no evaporation will take place in 
such a vacuum unless the water be warmer than 101° F., provided 
