Vlll PREFACE. 



By comparing attentively the chemical facts which 

 relate to this function with certain pathological ap- 

 pearances, and considering both in connection with 

 the actual structure of the respiratory organs, he was 

 induced, not only to reject the sufficiency of the ex- 

 planations which have been hitherto proposed, but 

 to form some opinions on the subject more conso- 

 nant, as it appeared to him, with the real designs of 

 nature. In the course of this investigation, analogy 

 readily suggested to him a comparison of the facts 

 ascertained in human respiration with those which 

 have been observed in the respiration of the inferior 

 animals ; and, from the lowest order of animal be- 

 ings, the transition to the analogous phenomena 

 which occur in the vegetable kingdom, was natural 

 and obvious. Thus, in a descending series, all the 

 great classes of animated nature were successively 

 brought under his review j and, arriving ultimately 

 at the most simple form of existence, he was led to 

 make it the first subject of investigation, and then 

 to retrace his steps through the more complex and 

 perfect forms of vegetable and animal life. 



It has been the constant aim of the author to make 

 observation and experiment the basis of all his rea- 

 sonings, and to deduce his conclusions from a full 

 and distinct consideration of all the circumstances 

 which seemed necessarily to affect them. The facts 



