INTRODUCTION. 43 



lourless or straw-coloured, but is often tinged by some colouring principle of 

 the plant ; it is transparent and brittle, very soluble in water, forming muci- 

 lage, but insoluble in alcohol. It is principally obtained from different species 

 of Acacia, but is plentiful in the plum, peach, and cherry. There are several 

 kinds of gum, differing from each other in the relative proportions of Arabine 

 or Bassorine they contain. 



Mucilage is a modification of gum, differing from it in being precipitated 

 by neutral acetate of lead. It abounds in the roots of many plants, as the 

 mallow, &c; in the leaves of others, as the benne; in the pith of some, as the 

 sassafras ; and in the testa of the seeds of others, as in flaxseed. 



Pectin, or Pectic acid. Is a compound found in many vegetable juices. 

 When moist, it forms a transparent jelly, tasteless and imperfectly insoluble 

 in water; when dry, it is in the form of a translucent mass. To the presence 

 of this substance, the consistence of fruit jellies is owing. Its composition is 

 O a H 8 O 10 . 



Extractive. Under this name certain matters found in the juices of 

 vegetables, and apparently similar in properties, have been designated. They 

 are, however, almost always impure, being modified by other vegetable com- 

 pounds. Many of what have been called the neutral principles of vegetables, 

 belong to this class. . 



Hydrogen in greater excess than in Water. 



' Fixed Oils. These substances are very similar in composition to those 

 obtained from the animal kingdom, and like them, are usually composed of 

 oleine, stearine, and margarine. They are principally obtained by expres- 

 sion from the fruit, and are mostly combined with a certain portion of muci- 

 lage and colouring matter, and hence acquire different colours and flavours. 

 When perfectly pure, they are colourless, and almost tasteless and odourless. 

 Many of them are highly esteemed as remedial agents. Their consistence 

 is various, from that of a thin fluid, to that of compact fat. Almost all of 

 them contain a peculiar acid, or are capable of acidifying. 



Volatile Oils. These are exceedingly numerous in the vegetable king- 

 dom, and are not confined to any particular part of the plant, but may occur 

 in all portions of them. They are characterized by possessing some strong 

 and peculiar odour, and are obtained by distilling the plant with water. 

 When perfectly pure, they are colourless, but in general, are coloured from 

 the presence of some impurity. Their odour is powerful, and their taste 

 pungent and burning. Many of them consist only of carbon and hydrogen, 

 whilst others contain some oxygen, and some are also combined with sul- 

 phur. To the first of these belong oil of turpentine, whose formula is C 5 H 4 ; 

 to the second, camphor, C 10 H 5 O ; to the third, oil of mustard, C 33 H 20 S 5 O 5 . 



Resins and Balsams. Resins exude spontaneously from many trees, and 

 are very various in composition. They are all insoluble in water, but solu- 

 ble in alcohol. Balsams consist of resin, volatile oil, and sometimes contain 

 benzoic acid. What are termed gum-resins, are composed of resin, volatile 

 oil, and gum. 



Caoutchouc. This substance when pure, is nearly white. It is softened, 

 but not dissolved by boiling water, nor is it soluble in alcohol, but it is so by 

 ether, naphtha, and some of the volatile oils. It is found in larger or smaller 

 proportions in most plants having milky juices. 



