RESINS. • 211 



a portion of camphor has been added, constitutes 

 opodeldoc. 



. 532. The resins, which are not less numerous than 

 the oils, resemble them in chemical composition and 

 combustibility; they are formed in large quantity by 

 many plants, and are very frequently seen adhering 

 in large transparent drops to the bark of trees which 

 produce them. Resins frequently look very much 

 like gum, but they may readily be distinguished from 

 gum by the fact that they are quite insoluble in water, 

 whilst- gum easily dissolves or softens in that fluid 

 (335). 



533. Most of the resins as they are obtained from 

 the trees which yield them are fluid, or at least soft, 

 and contain a portion of volatile oil; this is separated 

 by heat, and then the pure resin is left. Common 

 turpentine, which is a natural exudation, consists of 

 resin and oil of turpentine ; it is distilled, and the 

 oil of turpentine is thus procured, whilst the resin 

 called colophony is left. A healthy pine-tree yields 

 from six to twelve pounds of turpentine annually. 



634. When resinous woods are distilled or burnt, 

 they yield tar. The trees are commonly cut down 

 and burnt, so that the heat given out melts the resin 

 in the wood ; and that which escapes burning, runs 

 down and is collected below. 



535. In the manufacture of tar, the billets to be 

 burnt are piled into a stack, and then covered over 

 with turf; the combustion of the wood is thus smoth- 

 ered and kept down, and the loss of tar is prevented. 



