16 BULLETIN 101, U. S. DEPARTMENT OF AGRICULTURE. 



Species in Class IV are admirably adapted for this class of timbers. 

 The presence of radial resin ducts with the easily penetrated wood 

 structure makes possible the penetration radially of a large portion 

 of the volume of such timbers. 



If there is considerable difference in the penetrability of heartwood 

 and sapwood, dimension timbers are best treated when all of the sap- 

 wood is removed. There is no advantage in protecting parts of a 

 timber by heavily treated sapwood if other exposed parts have only 

 the lighter treatment possible in the heartwood. Such treatments 

 result in a practical loss of most of the oil absorbed by the sapwood 

 which may represent a very large proportion of the cost of treating 

 this kind of timber. A system of selecting timbers in accordance 

 with their suitability for treatment may prove very profitable to the 

 user of such timber. 



PAVING BLOCKS. 



These experiments indicate that with the possible exception of 

 Alpine fir and the heartwood of Tamarack. 1 a fairly thorough pene- 

 tration of the conifers in any form less than 12 inches in length 

 can usually be obtained. This fact is significant as regards treat- 

 ment of paving blocks. However, since the springwood. as a rule, 

 can not be thoroughly treated, woods with wide springwood bands 

 should not be used for this purpose. Other factors, such as strength 

 or wearing qualities, will of course greatly limit the number of species 

 suitable for paving blocks. 



THEORY OF PENETRANCE. 



It is a disputed question whether the cell walls under treatment are 

 permeable to creosote, or whether they contain openings through 

 which the oil passes. The first supposition does not account for the 

 very rapid penetration of certain species, nor does it account for the 

 greater ease of penetration in the thicker walled summerwood tra- 

 cheids than in the tliin-walled springwood tracheids. As regards 

 the second supposition, it has been suggested 2 that the minute slits 

 which frequently occur in the cell walls of seasoned wood are the 

 openings through which the oil passes. Seen in transverse section, 

 these slits appear as V-shaped openings extending about midway 

 through, the wall. They extend spirally around the cell and there is 

 the possibility that where the slits in adjacent walls cross each other 

 the cell wall is sufficiently broken clown to allow the passage of oil. 

 Another possibility is that a rupture occurs in the pit membrane 

 brought about by internal stresses of the wood during seasoning or 

 otherwise. While it seems that such ruptures are likely to occur, 

 there is little or no evidence to support this hypothesis. 



1 Tamarack blocks will take a good treatment in the green condition after steaming. 

 - The Physical Structure of Wood in Relation to the Penetrability, by H. D. Tiemann ; 

 Bull. 120, Am. Ry. Eng. and Main, of Way Association, January, 1910. 



