OF THE WOOD. 



In some trees, a number of the outermost rm^s difFer 

 greatly in colour from the innermost, and are called by 

 workmen the sap. In the Laburnum the former are 

 yellow, the latter brown. In the Oak and many other 

 trees a similar difference, though less striking, is percep- 

 tible, and in most the external rings are much less firm, 

 com lact, and durable than the rest, retaining more vital 

 principle, and more of the peculiar juices of the plant. 

 Such rings are all comprehended by Du Hamel under 

 the name of ^M/^ier, alburnum ; and he rightly observes 

 that this difference often extends to a greater number of 

 rings on one side of a tree than on another. It seems 

 that the more vigour there is in a tree, or side of a tree, 

 the sooner is its alburnum m ide perfect wood. By this 

 term, however, is properly understood only the layer of 

 new unhardened wood of the present year. When the 

 word alburnum is used in the following pages, it applies 

 to this part only. 



Physiologists have long differed and do still differ 

 about the origin of the wood. Malpighi and Grew 

 thought it was formed by the bark, and the best obser- 

 vations have confirmed their opinion. Hales supposed 

 the wood added a new layer to itself externally every 

 year. Linnaeus had a peculiar notion, that a new layer 

 of wood was secreted annually from the pith, and added 

 internally to the former ones. Truth obliges us to con- 

 fess that the latter theory is most devoid of any kind of 

 proof or probability. 



Du Hamel, by many experiments, proved the wood 

 to be secreted or deposited from the innermost part of 

 the bark or libe}-. He introduced plates of tinfoil under 



