PROXIMATE PRINCIPLES. 159 



Aromatic plants are much more common in hot, than cold 

 countries ; most of our aromatic spices are found in the 

 equatorial regions. 



Wax is found on the surface of the fruit of the bay-berry 

 (MYRICA cerifera). Beeswax is an animal production, made 

 by the bees from pollen or farina of plants. 



Camphor has much analogy with the volatile oils ; it is an 

 extract from the LAURUS camphora, or camphor tree of Japan. 



RESIN exudes from the pine, and some other trees ; it is dry, 

 insoluble in water, but soluble in alcohol and very inflamma- 

 ble. 



The people in new countries often use, as a substitute for 

 lamps, pine knots, which, abounding in resin, burn with a 

 bright flame. 



The difference between resin and the volatile oils, appears to 

 consist in the action of oxygen upon the resin ; for the oil in ab- 

 sorbing oxygen from the air, passes into the resinous state. 



Resins mixed with volatile oils form balsams ; thick, odor- 

 ous, and inflammable substances of this kind are the balsam 

 copaiva, dragon's blood, which, notwithstanding its terrific 

 name, is but the simple extract of a plant, (!)RAC(ENA draco ;) 

 the balsam of Tolu is the extract of the TOLUIFERA balsamum. 



These resins are sometimes mixed with gums, they are then 

 called gum-resins ; of this kind are gamboge, assafoetida, guaia- 

 cum, aloes, an extract from the ALOE perfoliata. These gum- 

 resins in flowing from vegetables are sometimes white and liquid 

 like milk, but they usually become brown and hard by expo- 

 sure to the air. 



Indian rubber* or as it is sometimes called, gum elastic, is 

 the product of a South American tree, (called the SIPHONIA 

 elastica,) an East Indian plant, (the URCEOLA elastica,) and 

 some other trees in the equatorial regions ; by exposure to the 

 air it hardens, becomes brown, and takes the appearance of 

 leather ; it can neither be dissolved by water nor alcohol. 

 The juice of the milk weed is said to be similar to that of the 

 Siphonia elastica, and that of other plants from which the In- 

 dian rubber is made.t 



The green principle. It is to this principle that all the green- 

 parts, exposed to light, owe their colour ; it undergoes changes 

 in the different states of the plant, in autumn becoming brown 

 or yellow. Davy attributes the change of colour to the form- 

 ation of some acid ; you know that a drop of sour wine, lemon 



* Caoutchouc. 



t Mr. H. Eaton, assistant professor at the Rensselaer Institution, prepared a 

 small quantity of the juice of the milk weed (Asclepias) in such a manner that 

 it could not be distinguished from the imported Indian rubber, either in external 

 appearance, or in its properties. 



