DURABILITY. 81 



the heavier wood is always the more durable. The amount of 

 woody substance a wood contains is also important, so that dark 

 summer- wood is more durable than light spring- wood. As 

 regards ring-pored woods, such as oak, ash and elm, broad-zoned 

 wood with comparatively narrow porous zones and with small 

 pores is more durable than narrow-zoned wood. Thus, for 

 instance, a cask made of narrow-zoned porous Spessart oak 

 seldom lasts without repairs more than 10-15 years, whilst a 

 broad-zoned oaken cask, the wood of which was grown near the 

 Rhine or Moselle, or in France or Hungary, may last for 30-40 

 years and more. In the case of conifers, on the contrary, as a rule, 

 narrow-zoned wood is more durable than broad-zoned wood. 



A. Mayr states that intense coloration of heartwood is also a 

 measure of the durability of timber. Faintly coloured heartwood 

 resembles sapwood in its properties, surpassing it only owing 

 to its superior dryness. The colouring matter of heartwood is 

 antiseptic, but intense heat and light are required to produce 

 tannin, so that woods from the south are most durable. 



Whatever local circumstances may increase the specific gravity 

 of wood of a certain species, will also increase its durability. 

 Thus, heavy coniferous wood of the lower and middle Alpine 

 zones is more durable than light coniferous wood grown in warm 

 lowlands ; on the other hand, heavy oakwood from the vine-region 

 of Europe has been proved by experience to be more durable 

 than oakwood from colder localities and poor soils. As regards 

 most broad-leaved trees, more durable wood is produced in the 

 open than in dense woods. This is directly the result of the 

 increased effects of light on the density of wood, and its truth 

 has long been proved by experience. {Vide Plates I. and II.) 



(b) Wood-sap is, as has been already stated, the most frequent 

 cause of the entrance of wood-destroying fungi. The possibility 

 of the greatest durability of which any wood is susceptible 

 therefore depends on the tree being felled when it contains least 

 sap. It is well known that the amount of sap in a tree varies 

 with the species, and that as a rule, broad-leaved trees contain 

 more sap than conifers, and as regards parts of trees, that the 

 sapwood usually contains far more sap than the heartwood. The 

 amount of sap also depends on the extent of the root-system, 

 and to a great extent on the season ; in general, spring and 



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