ECONOMIC WOODS OF THE UNITED STATES 59 



logs, poles, and posts. If seasoned too rapidly hardwood timbers 

 may split entirely open so as completely to destroy their value. 

 In handling such material it is a common practice to forestall 

 such checking by driving in <S-shaped metal wedges across the 

 incipient cracks. Such damage can also be reduced by more 

 careful piling and handling of the material. 



References 



Roth, F.: Timber, Bui. 10, U. S. Div. Forestry, pp. 32-37. 



Boulger, G. S.: Wood, pp. 80-88. 



Von Schkenck, H.: Seasoning of Timber, Bui. 41, U. S. Bu. of Forestry, 1903, 



p. 48. 

 Tiemann, H. D.: Effect of Moisture on the Strength and Stiffness of Wood, 



Bui. 70, U. S. Forest Service, pp. 76-79; 116-118; 123. 

 Baxer, Hugh P.: A Study in the Shrinkage of Wood (unpublished thesis, 



Yale Forest School, 1904). 

 Cahrens, C: Zur Kenntniss der innern Structur der vegetabilischen Zell- 



membranen, Jahrb. fur wissensch. Botanik, Vol. XXIII, 1892, pp. 



567-636. 

 Nageli, K. : Ueber den innern Bau der Vegetabilischen Zellenmembranen, 



Sitzb. d. Akad. Wiss., Miinchen, 1864, Pt. 1, 282-326; Pt. 2, 114-170. 



HYGROSCOPICITY 



Wood substance has the property of absorbing moisture from 

 the atmosphere. When artificially dried wood is exposed to the 

 open air it will increase in weight, due to the addition of hygro- 

 scopic water. Although the amount of water thus attracted is 

 always greater than in the surrounding air, it does not remain 

 constant, but varies with the humidity, and is equal to 8 to 16 

 (average 12) per cent of the dry weight of the wood. These 

 variations are accompanied by proportionate changes in volume, 

 that is, the wood alternately shrinks and swells, or "works." 

 Hygroscopicity can be reduced, but not entirely eliminated, by 

 subjecting wood to boiling, steaming, prolonged soaking, or 

 exposure to high temperature. 



This property of wood is a serious hindrance to its use in 

 certain positions where exact fitting is permanently desired. 

 Drawers and doors "stick" in damp weather, and become loose 

 in dry weather, or when artificially heated and dried for con- 



