GUMS, RESINS, OLEO-RESINS, ETC. 145 



of the graduations which she presents to our notice 

 tax to the uttermost our powers of classification. 

 This is well illustrated in the series of substances at 

 present under consideration, for which no one com- 

 prehensive name, nor any entirely satisfactory scheme 

 of classification, has as yet been suggested. The 

 names ' Gum ' and ' Balsam ' have been very loosely 

 employed in the past, and there is, in fact, an in- 

 sensible gradation from the limpid essential oils to 

 the solid resins. The essential oil known as Attar of 

 Rose is itself solid at ordinary temperatures, and re- 

 ference has already been made to the solid stearop- 

 tenes present in varying proportions in many others 

 of that series. When the resin is so incompletely 

 dissolved in the essential oil as to form a viscous 

 body, it is termed an oleo-resin, and it has been pro- 

 posed to confine the term * balsam ' to those fragrant 

 substances that contain cinnamic, or some analogous 

 acid, in addition to the volatile oil and resin of which 

 the turpentines alone consist. ' Gums,' properly so 

 called, are exudations from plants, soluble in water, at 

 least in part, forming with it a mucilage insoluble in 

 alcohol of 60 per cent, convertible by sulphuric acid 

 into dextrine, and with nitric acid yielding mucic and 

 oxalic acids. They are quite amorphous, being neither 

 organized like starch, nor crystallizable like sugar. 

 Resins, on the other hand, are insoluble in water, but 

 mostly soluble in alcohol, essential oils, ether, or 

 heated fatty oils; non-crystalline, incapable of subli- 

 mation, burning with a bright but smoky flame, con- 

 taining little oxygen and no nitrogen. A typical 

 resin is a pale yellow, transparent solid, with a glass- 



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