246 BELL SYSTEM TECHNICAL JOURNAL 



This picture of moisture adsorption seems to provide a reasonable 

 explanation for a variety of practical problems. 



Thus it is known that if fibrous materials, such as wood, paper or 

 fiber board are dried below a certain critical moisture content, perma- 

 nent changes may be expected in the structure; even serious damage 

 may result from too thorough drying at low temperatures. It seems 

 evident that when the moisture content of such more or less dense 

 structures is reduced to a point where the outer layers have less than 1 

 per cent of moisture, the internal surfaces of these layers may begin to 

 lose the monomolecular layer of water. The valence forces of surface hy- 

 droxyl groups are now no longer satisfied by water molecules, so that such 

 hydroxyls on contiguous surfaces may stick together. On readsorption 

 of moisture, portions of these surfaces may be so permanently attached 

 to one another that swelling will no longer occur in just the same way 

 as originally, and cracking and warping may result. 



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