PROPERTIES OF WOOD. 



7. Hygroscopicity. 



The hygroscopicity of a wood is its reaction to water and 

 water-vapour. If absolutely dry wood is placed in contact 

 with air saturated with water-vapour, as an organised body, 

 the walls of its tissues gradually absorb so much moisture 

 that the wood becomes saturated. The weight of the woody 

 tissue is then about 15 per cent, more than its absolutely dry 

 weight. If the wood is in air with a relative humidity 

 of 50, it absorbs gradually only 50 per cent, of the 

 water that it could absorb in saturated air, viz., about 7 to 

 8 per cent, of its own weight. The absorption of the walls 

 of woody tissue is therefore proportional to the relative 

 humidity of air, allowing a sufficient time for the action of the 

 ;itinospheric moisture. Water can be deposited only in drops 

 in the cell-lumina, which are completely surrounded by ligneous 

 walls, if the temperature of the air inside the cells is cooled 

 down almost to the dew-point owing to the cooling of the 

 external air; as the temperature rises, water disappears again 

 from the lamina. The water, which persists in the wood for 

 some time at least, at all temperatures, is either the remains of 

 the original water in the growing tree or has entered the 

 wood after contact with water, so that the air from the cell- 

 luinina is gradually replaced by water and the wood then 

 becomes saturated. 



In wood-industries, the importance of the saturation of wood 

 by water is not due to any consequent increase in weight, but 

 because wet wood is more accessible to fungi; also because 

 in many other technical qualities, such as transverse or 

 strength and combustibility, it becomes deteriorated and that 

 its shape alters as its contained moisture varies. The conse- 

 quent increase in volume of wet wood is termed swelling, 

 while a decrease in volume of drying wood is termed shrinking, 

 bo tli these actions being included in the word warping. 

 Shrinking is often accompanied by cracks, which cause 

 further deterioration in the quality of the wood. 



As already stated, absolutely dry wood may absorb water 

 from saturated air till it has increased 15 per cent, in weight, 

 wh.-n tho coil-walls are saturated, The expansion in volume 



