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.