SECTION XIV 

 RESINS 



Under the term resin a number of substances possessing certain 

 properties in common are grouped together, but it is impossible to 

 define the group as sharply as those of the fats, waxes, &c. The 

 most important of these properties is insolubility in water, but ready 

 solubility in alcohol, the alcoholic solution leaving on evaporation a 

 film of transparent varnish. Resins are hard, amorphous, brittle 

 solids (occasionally semi-solids), which soften when heated, and 

 finally melt to a clear, adhesive fluid ; they contain a large percentage 

 of carbon, but only little oxygen, and burn with a smoky flame ; they 

 are free from nitrogen. Many are soluble in ether or in chloroform, 

 but only a few dissolve in petroleum spirit. 



Although most resins, and indeed all resins of medicinal or 

 technical importance, are natural products, it is possible to produce 

 resins artificially. Various volatile oils, especially those containing 

 terpenes, yield resins by oxidation, and resinous bodies are also 

 formed by the action of potassium hydroxide on aldehydes, &c. 



Natural resins are commonly produced by special cells and 

 discharged, together with other products, into a cavity formed by 

 the separation of the cells from one another ; in this way the oleo- 

 resins of the Coniferce, the gum-resins of the Umbelliferce, &c., are 

 produced. The tissue intervening between oleo-resin ducts frequently 

 breaks down, so that branching cavities are formed ; in this case the 

 cell- walls appear to be converted into resin or oleo-resin, but the 

 quantity thus produced is seldom large. 



Guaiacum resin, on the other hand, is secreted in the vessels and 

 cells of the heartwood, completely filling them and thus obstructing 

 the passage of water. The resin of Indian hemp is secreted by 

 external, stalked glands, whilst shellac is probably produced by the 

 lac insect. 



In addition to the secretion ducts normally present in the plant, 

 others may be formed as the result of injury, and this may also take 

 place even if the plant produces normally no such ducts. The 

 number of the ducts thus formed may be very large and produce 



