HYGBO8COPICITY. 



67 



is, however, not thus completed. It has been shown by 

 observations, including those of Mayr, that when w T ood 

 remains lying for a long time in water a still further expan- 

 sion results, until the wood is completely saturated. If we 

 term the volume of wood the cell-walls of which are saturated 

 with water, such as the sap wood and heartwood of a freshly- 

 felled tree, the volume of green wood, then this increased 

 volume may be described as the saturated volume. 



Wood-tissue for some time in contact with air with a rela- 

 tive humidity of 50 per cent, swells up to a condition which is 

 half the swelling resulting from contact with saturated air 

 (relative humidity 100). However variable the absolute amount 



a Section of the cell-walls in wood absolutely dry. formal of niveelhe without 

 intermediate -paces. 1, The same, in air with relative humidity ;,(), the 

 intermediate spare* between the myeelhe tilled with water. < The game 

 in saturated air. or as ^reeii wood, saturated with water. 



of swelling may be in different species of wood, in all woods 

 the amount of swelling or shrinking is proportional to the 

 increase or decrease in the relative atmospheric humidity. 



The best and most natural way to understand the processes 

 of swelling and shrinking is to consider the cell-wall as com- 

 posed of mycella? ; when it is absolutely dry, adjoining 

 mycellffi, which though invisible must have a prismatic or 

 cubical shape, have no spaces between them (Fig. 29 a). 



If such a piece of cell-wall should come in contact with 

 moist air or with water, the water forces its way between the 

 mycellae, forming interstices, until the wall swells so as to 

 correspond to its saturated volume. Fig. 29 b shows this 

 saturation up to 50 per cent., 29 c., with saturated air. The 



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