OILS, BBSINS, ETO. 63 



ai'e usually thin-walled and filled with granular protoplasm. 

 The secretions are in some cases collected in drops in the 

 cell-cavity, in others they are caused to pass through the 

 cell- wall, while in stiU other instafices the cell- wall ruptures, 

 and permits the escape of the secreted matter. 



87. — There are three classes of essential oils, distinguished 

 by their chemical composition, as follows : 



(a) The pure hydrocarbons ; these are represented by the 

 formula C,„ H„. Oil of turpentine, obtained from the crude 

 turpentine of various Conifers, is the type. Oil of lemons, 

 oil of caraway, and oil of thyme are also of this class. 



(5) The oxidized essences, in addition to carbon and hy- 

 drogen, have oxygen in their composition. Of this nature 

 are camphor (C,„ H^^ 0), essence of cinnamon, essence of 

 wintergreen, etc. 



(c) The sulphuretted essences contain sulphur. To this 

 class belong the essential oils in mustard, horseradish, and 

 other Cruciferse, in onions, garlic, asafcetida, etc. That in 

 garlic, which may be taken as the type, is a sulphide, 

 ([CjHJ,, S), while that of the mustard is a sulpho-cyanide 

 (C^H^GNS). 



88. — Eesins are much like the essential oils in composition, 

 and are generally associated with and dissolved in them. 

 When separated from the essential oils by heat, the resins are 

 transparent or translucent brittle solids, insoluble in water, 

 but soluble in alcohol. Common rosin, which is the resi- 

 due left when the crude turpentine derived from several spe- 

 cies of pines is distilled with water, may be taken as the type. 

 It is an oxidized hydro-carbon, i.e., it contains carbon, hy- 

 drogen, and oxygen. 



89. — Gums. Under this name many different kinds of 

 products are commonly included. Some of them are with- 

 out doubt related to the resins, while others are allied to 

 starch and sugar. Of the latter kind gum-arabic (0„ H^^ 0„) 

 is the type, and allied to it are cerasin (from tlie cherry), 

 bassorin (gum tragacanth), and vegetable mucilage, which 

 Is abundant in mallow roots. 



90. — Pectin, or vegetable jelly (C^, H„ 0, J, is related to 

 the foregoing ; it forms, when moist, a transparent jelly, and 



