DELATIONS OF WOOD TO WATER. 53 



contents as possible. As already stated, the direction per- 

 pendicular to the medullary rays is that through which liquids 

 pass least freely. The fact that oak-staves are split along the 

 radius of the stem, and that the medullary rays are therefore 

 parallel to their longer surfaces, renders them very impervious 

 to liquids contained in the casks. 



Certain kinds of oakwood, however, both broad and narrow- 

 zoned, of coarse structure, are permeable to liquids, and casks 

 made of them will leak. Beechwood is very permeable, and is, 

 therefore, useless for wine- and beer-casks. Experience is said 

 to have proved that wood felled in December is less permeable 

 by liquids than wood felled in the spring. 



3. Ilesidts of the llelations of Wood to Water. 



Air-dried wood is subject to variations in the quantity of water 

 it holds, according to the condition of the atmosphere or of the 

 media with which it is in contact. The results of these changes 

 are an increase in bulk in the wood when water is absorbed, and 

 a decrease in its bulk when it parts with water. This 

 phenomenon is of the greatest importance among the economic 

 qualities of wood, and the wood is then said to shrink and swell, 

 or both actions are described as warping. They are explained by 

 the power the cell-wall possesses of imbibing liquids : when 

 water is imbibed, the iinjrclhe of the cell-wall are forced apart, 

 and the wood swells ; when the wood parts with water, the 

 mycdlce come closer together, and the wood shrinks. 



(a) Shrinkage of Dried Wood. — The loss of water by wood is 

 a measure of the amount of its shrinkage, and this therefore 

 varies greatly with green, half-dried or air-dried wood. As 

 wood cannot begin to shrink until it has lost all water from the 

 lumina of its cells and trachea?, and this only takes place after 

 the walls of these organs have begun to part with water, the total 

 amount of shrinkage will about be the same for both spring- and 

 summer-wood, though the former dries and therefore shrinks 

 more rapidly than the latter. Sap wood of most woods and especi- 

 ally of oak shrinks more than heartwood and semi-heartwood. 



The amount of shrinkage in different woods is not proportional 

 to their specific gravity. It may be said in general, that heavy 



