«0 TECHNICAL PUOPEKTIES OF WOOD. 



The causes of this decomposition are fungi and animnU 

 (chiefly insects). There can he no douht that fungi are the 

 chief causes of the decomposition of all organic substances, and 

 this Hartig has shown in the clearest and most convincing 

 manner as regards wood. Fungi gain access into wood by 

 infection, partly by mycelia, hut chiefly by spores ; if cir- 

 cumstances favour the spread of the latter, the mycelia of the 

 fungi grow in and among the wood-cells and fibres and by 

 nourishing themselves on their walls and contents break-up 

 and destroy the woody tissues. The destructive powers of insects 

 will be dealt with presently. 



Wood is most subject to decay when it still contains sap. 

 AVoody fibre, freed from sap and from the reserve nutritive 

 material which plants store in their woody tissues, is almost 

 imperishable, dampness being absolutely necessary for the 

 growth of fungi. Sappiness of wood is also a great furtherance 

 of the attacks of insects, for they prefer reserve-materials to 

 pure woody fibre. Wood-sap consists of water in which various 

 substances such as starch, dextrin, sugar, colouring matters, 

 etherial oils, tannins, albuminous substances, &c., are dissolved, 

 or suspended in grains or crystals. 



It is well known that different woods are in different degrees 

 subject to decay, and that many woods, and the same wood 

 under different circumstances, possess different degrees of dura- 

 bility. The chief factors determining the durability of a wood 

 are its nature, its treatment since it was felled and until it is 

 utilized, and the external influences and media to which it has 

 been exposed after utilization. 



2. Xatio'c of the Wood. 



From what has gone before, it may be easily imagined that 

 the specific gravity of a wood is a powerful factor in determining 

 its durability, and after this comes the question of its sappiness 

 and its degree of soundness. 



(a) The specific gravity of a wood taken alone, affords no 

 conclusive argument as to its durability. Several light woods, 

 as for instance conifers, may be more durable than heavier woods 

 like beech, birch and maples, but in the case of the same species, 



