PREFACE Xi 



whole of this inquiry, to combine anatomical fact 

 with those reasonings which relate immediately to 

 the living system. Unfortunately for physiology, 

 this circumstance has not always been sufficiently 

 kept in view, and the laws which govern the move- 

 ments of inanimate matter have too frequently been 

 applied, without reserve or discrimination,' to explain, 

 the functions of organized and living beings. To a 

 certain extent, indeed, animated bodies are subject 

 to the same laws as all other material substances; 

 but the peculiar properties which they derive from, 

 the principle of life can never be explained on me- 

 chanical or chemical principles. Although, there- 

 fore, it will be found, that chemical phenomena have 

 received a due share of attention, yet they have at 

 all times been held in subservience to the establish- 

 ed truths of anatomy, and to those laws which pe- 

 culiarly belong to bodies possessed of the principle 

 of life. 



In the present Treatise, no reference whatever 

 has been made to the theories which have been 

 proposed to explain the phenomena of vegetation 

 and respiration. Such theories, it is evident, must 

 rest entirely on a" knowledge of the changes pro- 

 duced by living bodies on the air ; and it is the 

 professed object of this inquiry to examine, with 

 more attention than has yet been done, into the na- 



