4. The maximum or highest temperature to which the different species of wood may be exposed without serious loss of strength has not yet been determined. 5. The optimum or absolute correct temperature for drying the different species of wood is as yet entirely unsettled. 6. The inter-relation between wood and water is as imperfectly known to dry-kiln operators as that between wood and heat. 7. What moisture conditions obtain in a stick of air-dried wood? 8. How is the moisture distinguished? 9. What is its form? 10. What is the meaning of the peculiar surface conditions which even in air-dried wood appear to indicate incipient "case-hardening"? 11. The manner in which the water passes from the interior of a piece of wood to its surface has not as yet been fully determined. These questions can be answered thus far only by speculation or, at best, on the basis of incomplete data. Until these problems are solved, kiln-drying must necessarily remain without the guidance of complete scientific theory. 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 substance like a piece of cloth must be studied experimentally. It cannot well be worked out theoretically. SECTION X HOW WOOD IS SEASONED Methods of Drying The choice of a method of drying depends largely upon the object in view. The principal objects may be grouped under three main heads, as follows: 1. To reduce shipping weight. 2. To reduce the quantity necessary to carry in stock. 3. To prepare the wood for its ultimate use and improve its qualities. When wood will stand the temperature without excessive checking or undue shrinkage or loss in strength, the first object is most readily attained by heating the wood above the boiling point in a closed chamber, with a large circulation of air or vapor, so arranged that the excess steam produced will escape. This process manifestly does not apply to many of the hardwoods, but is applicable to many of the softwoods. It is used especially in the northwestern part of the United States, where Douglas fir boards one inch thick are dried in from 40 to 65 hours, and sometimes in as short a time as 24 hours. In the latter case superheated steam at 300 degrees Fahrenheit was forced into the chamber but, of course, the lumber could not be heated thereby much above the boiling point so long as it contained any free water. This lumber, however, contained but 34 per cent moisture to start with, and the most rapid rate was 1.6 per cent loss per hour.