ORAOKINfl ANT) SPLITTING DUBABILITY. 28 



is advisable to leave the bark on for a fev? months until the wood 

 has undergone a certain degree of, seasoning; or the bark should 

 be preserved for a few feet at the ends in order to secure a more 

 uniform drying throughout the length of the log ; or the bark 

 should be removed only in spiral strips running round the whole log. 

 Short round pieces, that are ultimately to be cut up, are effectually 

 preserved from cracks by sawing them through lengthwise along 

 a single line as far as the pith ; this is the way in which pieces 

 of box for engraving purposes are treated. A hole of- sufficient 

 diameter bored through the centre of the log also prevents the 

 formation of cracks. Sawn pieces are protected either by clamping 

 the ends, or by driving iron SS into the ends, or by tarring the 

 ends and pressing on tough brown paper before the tar is dry. 

 Steaming, followed by slow drying, also prevents cracks, or, at the 

 most, allows only a few small ones to form. 



Section VIII. — Dueability. 



By the durability of a wood we understand the resistance it 

 offers, when brought into use, to the various causes of decay and 

 to the attacks of insects and other animals. 



Decay. — Decay is the result of the ravages of various fungi, 

 which invade, by means of their fine thread-like mycelia, the 

 entire tissue of the wood, obtaining starch, saccharine matters, 

 nitrogenous substances, and inorganic elements, such as potassium, 

 phosphorus, calcium, &c., from the medullary rays, and other 

 food materials, such as water, air, mineral salts, tannin, coniferin, 

 lignin from the lignified walls of the cells, tracheides and vessels 

 everywhere. The structure of the wall is thus completely des- 

 troyed, and the entire mass of the wood becomes brittle and falls 

 easily into powder. As fungi cannot live without nitrogen, 

 wood could be made imperishable were it possible to rid it of all 

 the proteid substances present in the medullary rays. • Since fungi 

 require a considerable quantity of moisture, the use of thoroughly 

 seasoned wood in a sufficiently dry environment would effectual- 

 Iv prevent decay. So would complete and uninterrupted sub- 

 mergence in water deep enough not to be overcharged with 

 air preserve wood against decay. Indeed submergence for a 

 sufficiently prolonged period renders wood imperishable : during 

 the submergence slow chemical and physical changes go on, by 

 which the starch, sugars, nitrogenous matters, &c., are dissolved 

 out, and replaced by mineral deposits from the water, both the 



