54 TIMBER. 



exposure; the resin of pines is very viscid, their turpentines readily 

 oxidize and darken when brought in contact with air. Resins are 

 gathered more commonly either from cracks, such as "wind" and "ring 

 shakes," as in the case of larch and fir (Venetian turpentine), or else 

 from wounds made especially for this purpose, as in the case of naval 

 stores gathered from pines. This latter process is known as " bleed- 

 ing," "tapping," or "orcharding," and is at present the principal 

 method of obtaining turpentines and rosins. 



On burning resinous wood, wood tar, etc., in a smoldering fire, soot 

 is deposited on the walls and partitions of the specially constructed 

 soot pit. It is then collected, but must be freed of various products of 

 dry distillation, by carefully heating to red heat before it becomes the 

 lampblack used in printers' ink and otherwise much employed in the 

 arts. 



Many kinds of wood and the bark of most trees contain tannin. To 

 serve in tanning the bark must contain at least 3 per cent of tannin; 

 the kinds mostly used vary from 5 to 15 per cent, and even the best 

 probably never furnish over 20 per cent in the average. The use of 

 tan bark involves considerable disadvantages. It is difficult to dry and 

 preserve, very liable to mold, bulky, and therefore expensive to ship 

 and store, and very variable in the amount of tannin which it contains. 



To avoid these difficulties the tannic compounds are, in recent times, 

 leached out of the finely ground bark and wood, condensed by evapo- 

 ration, and shipped as extracts containing 80 to 90 per cent of tannin. 



The manufacture of pulp as well as the production of fiber capable 

 of being spun and woven, are also technological uses of wood, which 

 rely partly upon chemical reactions. 



VII.— DURABILITY AND DECAY. 



All wood is equally durable under certain conditions. Kept dry or 

 submerged, it lasts indefinitely. Pieces of pine have been unearthed 

 in Illinois which have lain buried 60 or more feet deep for many cen- 

 turies. Deposits of sound logs of oak, buried for unknown ages, have 

 been unearthed in Bavaria; parts of the piles of the lake dwellers, 

 driven more than two thousand years ago, are still intact. 



On the radial section of a piece of pine timber, with one of the 

 shelf-like, fungus growths, as shown in fig. 34, both bark and wood 

 are seen to be affected. A small particle of the half-decayed wood pre- 

 sents pictures like that of fig. 35. Slender, branching threads are 

 seen to attach themselves closely to the walls of the cells, and to pierce 

 these in all directions. Thus these little threads of fungus mycelium 

 soon form a perfect network in the wood, and as they increase in num- 

 ber they dissolve the walls, and convert the wood substance and cell 

 contents into sugar like food for their own consumption. In some cases 

 it is the woody cell wall alone that is attacked. In other cases they 



