ECONOMIC WOODS OF THE UNITED STATES 61 



it is impossible to force any air through the vessels, even for short 

 lengtns and with very high pressure, since in this case they are 

 blocked with tyloses. Even in the red or black oaks, however, 

 air cannot be forced through the other elements of green wood. 



When wood becomes dry its penetrability by both gases and 

 liquids is increased to a remarkable extent. The same specimen 

 of white oak which, while green, effectually withstood an air 

 pressure of 150 pounds per square inch will, when dry, allow the 

 passage of air, not only through the vessels, but also the other 

 elements, under a pressure of 5 pounds per square inch or less. 

 Similar effects are produced by drying any wood beyond its fibre- 

 saturation point. This fact emphasizes the great importance of 

 seasoning wood before attempting to impregnate it with pre- 

 servatives. 



According to Tiemann (loc. cit), the explanation of this 

 is that the drying of the cell walls causes minute checks or slits 

 to occur in the primary walls. The dryer the wood becomes the 

 larger the slits and the more permeable the wood. These slits 

 do not entirely close when the wood is resoaked, so that wood 

 once dried cannot be restored to its original condition. 



Steaming is said to produce similar results, though the slits 

 apparently are not as wide as when wood is air-dried. It is prob- 

 able, however, that the maximum amount of slitting would result 

 from thoroughly drying wood that had been previously steamed. 

 Boiling green wood in oil results in more or less seasoning of the 

 outer portions, thus allowing some penetration by the oil. 



Dry woods, however, differ greatly in penetrability. Light, 

 porous woods as a rule are much easier to impregnate than dense, 

 compact ones. Heartwood of any species offers more resistance 

 than the sapwood, due probably to the presence in the walls of 

 gums, resins, and other infiltrations. Tyloses, which always 

 reduce penetrability, are mostly absent from the outer portion of 

 sapwood even when very abundant in the heartwood of the same 

 tree. In the wood of Gymnosperms it appears that the wood- 

 parenchyma cells are more penetrable than the tracheids. Open 

 resin ducts permit the entrance of fluids into the body of the 

 wood, behaving in a manner similar to the vessels of Dicotyledons. 



The whole question of the penetrability of wood is extremely 

 important in view of the increasing interest in timber preserva- 

 tion, and comparatively little is definitely known regarding it. 

 Experiments by the United States Forest Service are now in 



