RESISTANCE OF HARDWOODS TO CREOSOTE. 9 
FACTORS AFFECTING PENETRATION. 
The penetrability of wood is greatly influenced by many factors 
not studied in these tests. Trees of the same species grown in differ- 
ent localities may differ greatly in their properties, hence the natural 
variability of the wood may largely influence penetration. The char- 
acter of the penetration is an important factor. So also is the mois- 
ture content of the wood. Timber which has just been cut and which 
is, therefore, in the green condition is, as a rule, very much more 
resistant to treatment than that which has had the moisture removed. 
The method of seasoning the timber may also have an important 
bearing on the resulting penetration and absorption of preservative. 
Thus, wood which has been air seasoned may give quite different 
results from that which has been artificially seasoned either by steam- 
ing, boiling in oil, or by other processes. 
In order to eliminate, so far as possible, variations arising from 
differences in moisture, the specimens used in the tests were dried 
to a low moisture content, although it is recognized that this is not 
done in commercial work where the methods of seasoning may vary 
greatly with different species of wood. Furthermore, sapwood and 
heartwood specimens were treated separately, although in commer- 
cial work timber under treatment may contain both heartwood and 
sapwood in widely varying proportions. 
It was not within the scope of this work to study each species 
separately under the conditions most suitable for its treatment. The 
study was confined to one tree of each species. ,. The trees were 
selected to be as nearly representative as possible, and all specimens 
used were closely matched. Since identical treatments were given to 
each species, it is possible to compare the penetrations and absorp- 
tions in the various species when tested under the same conditions. 
EFFECT OF STRUCTURE ON PENETRANCE. 
POSES OR VESSELS. 
The pores or vessels, which serve to furnish a channel for the 
passage of sap from the roots of the tree to the leaves, are a very 
important factor in penetration with wood preservatives, especially 
when they are open passages through the wood. 
1. Pores open. — In red oak, Plates II and III, the pores or vessels 
( V) are open passages. It is even possible to blow air through sticks 
of red oak several feet long. In a diffuse-porous wood they are more 
numerous but of smaller diameter. When the pores are open as in 
red oak or basswood, they are the main factor in the initial pene- 
trance of the wood. 
11961°— IS— Bull. 606 2 
