92 PROPERTIES OF WOOD. 



investigators have not calculated the limits of elasticity, but 

 have stated that the two moduli correspond. 



As regards specific weight, they affirm that for any species, 

 heavier wood is stronger, so for two species of spruce, the 

 heavier is stronger and more elastic. Schwappach goes more 

 into detail on the connection between specific weight and 

 transverse strength, stating that : 



Transverse strength, depends on : 



(a). Tree-parts. The external wood is the strongest, also 

 usually the heaviest. In the crown of a tree, sometimes the 

 weight, sometimes the strength is greater. The so-called 

 hard and heavy side of coniferous trees is weaker than the 

 so-called soft side. According to Foppel* also, the wood of the 

 upper side of branches is stronger and more elastic than that 

 of their lower side, so that lighter wood is stronger than 

 heavy wood. 



(b). Age. Old wood is stronger, though lighter, than young 

 wood ; in the Scots pine, specific weight diminishes after a 

 tree is sixty years old, but strength increases. 



(c). Locality. The strength diminishes as the tree is grown 

 away from its optimum climate (cf. p. 56). 



(d). Soil. The best soil produces the strongest wood ; it has 

 been shown already that the best soil does not always produce 

 the heaviest wood. 



(e). Wetness of the wood. Variations of 1 per cent, in wet- 

 ness give differences up to 8 per cent, in strength. 



Schwappach hence decides that specific weight alone is no 

 determining cause of strength. 



Only after eliminating the results of age, locality and method 

 of production, and after seasoning the wood, is its strength 

 dependent upon its specific weight alone. Thus in order to 

 avoid one mistake further investigations involving fresh sources 

 of error must be undertaken. Tetmajer states that the propor- 

 tion in which cellulose, lignin, gums, etc., are mixed in the cell- 

 wall does not affect specific weight, nor does the mode of 

 union of these materials with one another, nor that of adjacent 

 chills (cohesion). Tetmajer alleges that the amount of defor- 

 mation, which in tests of strength, occurs in displacement of 



* II. Hartig, " Holzuntereuchungen Altea o. Neues." r.crlin, r.xn. 



