42 WOOD AND FOB EST. 



The lighter kinds have tlie most water in the sap-wood, thus 

 syeamore has moi'e than hickorj'. 



Curiously enough, a tree contains about as much water in \\'inter 

 as in summer. The water is held there, it is supposed, by capillary 

 attraction, since the cells are inactive, so that at all times the water 

 in wood keeps the cell walls distended. 



Tin-: sin(iKK.-\.GE of wood. 



When a tree is cut down, its water at once begins to evaporate. 

 This ])rocess is called "seasoning."" In drying, the fi'ee water within 

 the cells keeps the cell walls saturated; but when all the free water 

 has been removed, the cell walls begin to yield up their moisture. 

 Watei' will not flow out of wood unless it is forced out Ijy heat, as 

 when green wood is put on a fire. Ordinaiily it evaporates slowly. 



The "\^'ater evaporates faster from some k'inds of wood than from 

 other kinds, e. g., from white pine than from oak, from small pieces 

 than from lai-ge. and from end grain than fron^ a longitudinal sec- 

 tion ; and it also evaporates faster in high than in low temperatures. 



Evaporation affects wood in three respects, weight, strength, and 

 size. The weight is reduced, the strength is increased, and shrinkage 

 takes place. The reduction in weight and increase in strength, im- 

 portant as they are, ai'e of less importance than the shrinkage, which 

 often involves warping and other distortions. The water in wood 

 affects its size liy keeping the cell walls distended. 



If all the cells of a ]iiece of wooil flere the same size, and had 

 walls the same thickness, and all ran in the same direction, then the 

 shrinkage would be uniform. But. as we have seen, tlie structure of 

 wood is not homogeneous. Some cellular elements are larrje. some 

 small, some have thick walls, some thin walls, some run longitudinallv 

 and some (the pith ravs) run radially. The elfects will be various 

 in differently shaped jiieces of wood but they can easilv he accounted 

 foj- if one bears in miud lliesc three facts: (1) tliat the shrinkao-e is 

 in the cell wall, and therefore (2) that the thick-walled cells shrink 

 mo7'e than thin-walled cells and (3) that the cells do nor shrink 

 much, if aiiy, lengthwise. 



(1) The shrinkage of wood takes place in the walls of the cells 

 that compose it, that is. the cell walls become thinner, as indicated 

 by the clotted lines in Fig. .3.x which is a cross-section of a sino-lo cell. 



'See Eandworh in ^Yood, Cliapter III. 



