CHAP. I. MECHANICAL PROCESSES. 3/9 



trived to accelerate the operations of nature by 

 means of artificial aid in the application of the 

 wimble or ax, widening the passages which the ex- 

 travasated fluid has forced, or opening up a new 

 one. The plant bleeds now more freely, and the 

 substance wanted is obtained in abundance ; whether 

 it is the gum of the Cherry-tree, the resin of the 

 Fir, the manna of the Ash, the opium of the Poppy, 

 or the sap of the Birch or Maple. 



But it more frequently happens that the process Artificial. 

 employed is wholly artificial, and altogether effected 

 without the operation of natural causes. When 

 the juices are enclosed in vesicles lodged in parts 

 that are isolated, or may easily be isolated, the 

 vesicles may be opened by means of rasps or 

 graters, and the juices expressed by the hand, or 

 by some other fit instrument. Thus the volatile oil 

 may be obtained that is lodged in the rind of the 

 Lemon or Cedrela. When the substance to be ex- 

 tracted lies more deeply concealed in the plant, or 

 in parts which cannot be easily detached from the 

 rest, it may then become necessary to pound or 

 bruise the whole or a great part of the plant, and to 

 subject it thus modified to the action of a press. 

 Thus seeds are sometimes treated to express their 

 essential oils. And if by the action of bruising 

 or pressing heterogeneous ingredients have been 

 mixed together, they may generally be separated 

 with considerable accuracy by means of decanta- 

 tion, when the substances held in suspension have 



