of the Corneous purls of Insects. Ill 



truly philosophical observers, vtIio Avill never found zoological 

 aifinities on characters so inconstant as the nature of the salts 

 which enter into the composition of the animal organs. Do we not 

 know that these bodies are subjected to the influence of external 

 causes of every kind ; that their composition varies with the habi- 

 tation, food and age of the respective individuals, and perhaps 

 from a number of other circumstances ? Were zoological affinities 

 to be founded on the chemical nature of the animal organs, instead 

 of seeking them in inorganic materials which vary from many 

 causes, and belong exclusively to no particular organs, but arc 

 found, either habitually, or accidentally in almost all, would it 

 not be more rational to found them on the nature of the animal 

 substance which, in truth, constitutes the organ, because it is the 

 product of vital action ? But, in my opinion, the chemical com- 

 position is incapable of confirming the established laws of zoology ; 

 its proper office seems to be to furnish us with facts calculated io 

 enlighten physiology concerning the formation and functions of 

 the organs. It is with this sole end that I undertook my present 

 labours, which are to be regarded merely as the point of departure 

 for subsequent researches, and as a feeble essay, in comparison 

 with what yet remains to be done on this subject. 



Remarhs on the preceding Paper. 



The results of the experiments detailed in the foregoing paper 

 seem to illustrate the beautiful gradations that prevail throu<'h the 

 whole range of animated nature, and to establish as it Mere a con- 

 necting link between the vegetable and the animal kingdoms. One 

 of the most striking diHerences, in the chemical con)position of 

 animal and vegetable substances, is the general prevalence of nitro- 

 gen in the former, as one of tlieir constituent elements, and the 

 want of it in the latter. It has, how^'ver, been long Known that 

 the rule is not in either case without exception ; thus nitrogen con- 



