158 PHYSIOLOGICAL VIEWS. 



Citric acid is found in the lemon ; it is mixed with the malic 

 acid in the gooseberry, the cherry and the strawberry. 



Quinic acid is obtained from the Peruvian bark (Cinchona). 



Gallic acid is obtained from the oak, and the sumach ; it is 

 highly astringent. 



Benzole acid is found in the LAURUS benzoin, and in the Va- 

 nilla ; this is highly aromatic ; it is thought to give the agreea- 

 ble odour to balms. 



Prussic acid ; this acid gives out a strong odour like bitter 

 almonds ; it is an active poison ; it is obtained from peach 

 meats and blossoms, from bitter almonds, &c. 



The 2d order includes gums, sugar, &c. 



The Gums. Of these there are many kinds ; they have 

 neither taste nor smell : dissolved in water, they form a muci- 

 lage more or less thick. The principal gums are, 



Gum Arabic, which flows from the plant MIMOSA nilotica ; 



Common Gums, such as issue from the peach tree, the cher- 

 ry tree, and many others. 



Sugar is a substance which dissolves in water, and has a 

 sweet taste , it is obtained from the sugar cane, the sugar ma- 

 ple, from the stalks of indian corn, pumpkins, beets, and sweet 

 apples. All vegetables which have a sweet taste may be 

 made to yield sugar. 



The 3d order includes oils, wax, resins, &c. 



Oils. These are fluid and combustible substances, which 

 do not unite with water. They are divided into Fixed and 

 Volatile. The fixed oils are thick, and have little odour. 



The oil of sweet almonds, and olive oil, grow thick and 

 opaque by being exposed to the air. 



The Oil of Flax Seed, called linseed oil, and some other oils, 

 dry without losing their transparency ; it is this quality which 

 renders linseed oil so valuable to painters. 



The Volatile oils are distinguished from the fixed oils by 

 their aromatic odours, and their tendency to fly off, from which 

 circumstance the term volatile is derived. Some of these oils 

 are those of the orange, lavender, rose, jasmine, peppermint, 

 and wintergreen. These oils are sometimes greatly reduced 

 by being mixed with alcohol, and are then called essences. 

 The volatile oils may be found in a great variety of plants, par- 

 ticularly those of the Labiate family. 



The Aroma, or aromatic property, consists chiefly of the 

 odours which are exhaled from plants, containing volatile oil ; 

 it is this oil which throws out the aromatic odour of the ginger 

 plant, of the myrtle, rose, and other sweet scented plants. 



Second order Third order What substances besides oils belong to th 

 third order of the fir.u class of proximate principles ? 



