12 TIMBER AND TIMBER TREES. [CHAP. i. 



spread over a larger surface. The sap-wood is, as a rule, 

 darker in the white-wood class than the heart-wood, 

 whether seasoned or unseasoned, but is paler in colour 

 in most hardwood trees which have had time to season. 

 In some of the white, or softer woods, when fresh cut, 

 the difference is scarcely perceptible ; but exposure to 

 the air quickly gives to the outer layers a greenish tinge, 

 as a species of mildew attacks them. With, I believe, 

 only a few solitary exceptions, great care is taken to 

 remove all sap-wood from the scantlings under conver- 

 sion, if they are required for works of an important 

 character. 



