(34) 



be regarded as turpentine, modified and rendered impure 

 by partial burning. It is obtained from the same trees 

 that yield turpentine, but the dead wood and stumps are 

 preferred. The wood is stacked and so enclosed by earth 

 as to partly exclude air, and is then fired at the top. As 

 the wood burns above, the heat drives out the liquid tar 

 just below, which runs off into vats and is stored in barrels. 

 The charcoal powder which enters renders it black. A 

 more perfect method is to distil it in suitable retorts. By 

 subsequent distillation of the tar, oil of tar is driven off, 

 naval pitch being left behind. Oil of tar contains, or yields 

 a large number of valuable substances, such as Guaiacol, 

 creosol, naphthalene, toluene, and xylene. In addition 

 to the resins obtained from living kinds of trees, there are 

 fossil resins of different degrees of hardness and color; 

 these enter largely into the manufacture of varnishes. 

 Varnish is a solution of one or more resins in some volatile 

 liquid which, on evaporating, deposits a uniform and con- 

 tinuous layer of the resin upon the surface to which the 

 varnish was applied. Such a coating of varnish, if of good 

 quality, is both hard and tough, hence not easily scratched, 

 insoluble in water and waterproof, capable of taking a 

 high polish, but melting and burning readily. Varnish 

 resins differ in quality and value according to the degree 

 in which the varnish made from them possesses the proper- 

 ties named above. The best is probably anime copal. 

 Not only is it so hard and tough as to stand floor-wear, 

 but it is soluble in so few substances that the spilling of 

 most liquids upon it will not injure it. Some of the trees 

 yielding varnish resins are now almost or quite extinct, 

 and the lumps of resin have lain buried in the soil in a 

 fossil state since the age when these trees were living. 

 Plant Constituents. Cases 63 to 68. — This exhibit con- 

 sists of a series of alkaloids, acids, glucosides, amaroids, 

 albuminoids, resinoids, and enzymes. These substances 

 plants store up in their tissues, or in the tissues of one or 

 more organs, and from them they are extracted for use in 

 all branches of the arts, sciences, and industries. 



