314 CHEMISTRY. 



so widely diffused that they can be considered essential 

 constituents of plants every where ; while others appear 

 only in particular plants, and though essential, are not uni- 

 versally so. It is the former class, which may properly be 

 called the constituents of plants, that we shall speak of now, 

 reserving the consideration of the latter class for another 

 chapter. In treating of them, we shall speak of the changes 

 effected in them by the operations both of nature and of 

 art. 



429. "Wood. What is termed wood in chemistry is the 

 vegetable tissue which makes the framework of all vegeta- 

 ble growths in all their parts, giving to them their shape 

 and firmness. It is the solid part of all vegetable organs. 

 It is to plants what bones, muscles, tendons, skin, etc., are 

 to animals. Woody fibre is present even in the most deli- 

 cate and tender fruits, holding in its interstices the juices. 

 It is sometimes so exceedingly delicate that in crushing the 

 fruit there seems to be almost nothing but juice. In some 

 fruits, as the orange, the woody tissue is beautifully ar- 

 ranged in long and slender sacs or bottles containing the 

 fluid for our use, the sacs being packed into several differ- 

 ent compartments, and each compartment being made of 

 woody tissue. This same tissue, which is so soft and finely 

 divided in the pulp of fruits, in leaves, and flowers, is con- 

 densed and hard in what is ordinarily called wood, in bark, 

 in straw, and the husks of grain, and especially in the shells 

 of nuts and the stones of cherries, peaches, etc. The so- 

 called vegetable ivory is chiefly condensed wood. In cork 

 we have wood in a very light, porous, and elastic form. 



430. Cellulose. The essential part of woody fibre is called 

 cellulose; this has the composition C 6 H 10 O 5 ; it is nearly 

 pure in cotton, paper, and wood pulp, provided they are 

 not colored with any thing and are not starched. 



Pure cellulose may be obtained by washing white cotton, 



