wood. t>;> 



which can be suddenly dissipated only by the applica- 

 tion cf heat. This external portion of wood which is 

 named sap by the workmen, is for obvious reasons to 

 be rejected whenever durability and strength of mate- 

 rials are required. By Duhamel it was named Alburnum 

 inconsequence of its being uniformly white, whereas 

 the central layers are frequently of a different colour. 

 In the Guiacum they are black, in the Oak and Labur- 

 num they are brown, and in other examples they are 

 of a reddish hue. The exterior and unripe portion is 

 converted into perfect wood, much sooner in vigorous 

 trees than in others, and this change is sometimes ef- 

 fected earlier on one, than on the other side of the 

 same tree, inconsequence of its more vigorous growth. 



The term Alburnum which Duhamel employed in a 

 more extensive sense, is usually restricted to the exter- 

 nal layer, whose texture is softer and more abur. 

 stored with juices than the others. From this pecul- 

 iarity of external character, it was at one time thought 

 to be a substance essentially different from that of the 

 layers it invests. By the ancients it was regarded as 

 analagous to the fat of animals, and intended perhaps to 

 serve as nutriment during the winter. But it is now 

 known to be merely wood in a less condensed state. 

 being yet lighter and softer than the interior layers, 

 but acquiring strength and solidity with age. It does 

 not however acquire its utmost degree of solidity till 

 after a number of years ; but when divested of its bark 

 it is converted into perfect wood in a single summer/' 



It is generally believed that each of the concentric 

 layers is the growth of a single year, and that the age 

 of a tree may be known by the number of circular lay 



♦Keith, Vol. J. p. 331. 



