xists begins to
shrink, the result is the development of opposing stresses, the effect
of which is sometimes disastrous. The shrinkage around the knots seems
to be particularly uneven, so that checking at the knots is quite
common.

Some woods, such as Western red cedar, redwood, and eucalyptus, become
very plastic when hot and moist. The result of drying-out the free
water at high temperature may be to collapse the cells. The gums are
known to be quite soft and plastic, if they are moist, at high
temperature, but they do not collapse so far as we have been able to
determine.

The cells of certain species of wood appear to lack cohesion,
especially at the junction between the annual rings. As a result,
checks and ring shakes are very common in Western larch and hemlock.
The parenchyma cells of the medullary rays in oak do not cohere
strongly and often check open, especially when steamed too severely.

Unsolved Problems in Kiln-drying

1. Physical data of the properties of wood in relation to
heat are meagre.

2. Figures on the specific heat of wood are not readily
available, though upon this rests not only the exact
operation of heating coils for kilns, but the theory of
kiln-drying as a whole.

3. Great divergence is shown in the results of experiments
in the conductivity of wood. It remains to be seen whether
the known variation of conductivity with moisture content
will reduce these results to uniformity.

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 specu