68 PROXIMATE PRINCIPLES IN GENERAL. 



exclusively organic origin, but they resemble the latter in their crys- 

 tallizability and their definite chemical composition ; in consequence 

 of which their chemical investigation may be pursued in nearly 

 the same manner, and their chemical changes expressed in nearly 

 the same terms. But the proximate principles of the third class 

 are in every respect peculiar. They have an exclusively organic 

 origin ; not being found except as ingredients of living or recently 

 dead animals or vegetables. They have not a definite chemical 

 composition, and are consequently not crystallizable ; and the forms 

 which they present, and the chemical changes which they undergo 

 in the body, are such as cannot be expressed by ordinary chemical 

 phraseology. This class includes such substances as albumen, 

 fibrin, casein, &c. 



