~'2I) PHYSIOLOGICAL BOTANY. PART II. 



plants and sea weeds, and is largely prepared on the 

 shores of the Mediterranean for the European market. 



(i^ii.) Origin of extraneous Deposits. The various 

 other products, such as oxides, metallic salts, &c., which 

 occur in small quantities in the ashes of plants, have all 

 been either derived immediately from the soil or intro- 

 duced in some way by absorption from the atmosphere. 

 It seems clearly established that none of them ought to 

 be considered as the direct product of any vegetative 

 function, as was once supposed ; and it has been satis- 

 factorily shown that however carefully the experiments 

 may have been made which favour such a theory, and 

 however cautiously the means may have been taken for 

 excluding all foreign matters from access to the grow- 

 ing plant, error was unavoidable. The extreme mi- 

 nuteness of the elementary organs of plants, and the 

 more delicate nuinipnlutitms of a natural chemistry, are 

 capable of separating the minutest portions of foreign 

 matters from the materials with which they are brought 

 in contact, however carefully and accurately these ma- 

 terials may have been purified and cleansed by artificial 

 processes. It seems to be impossible for instance to 

 provide even distilled water so pure, but what some 

 traces or other of foreign matter may be detected in it. 



