INTRODUCTION. 



combination with inorganic substances, which, though per- 

 haps not absolutely necessary to the properties by which they 

 are recognized out of the body, are essential in the perform- 

 ance of their vital functions in the economy. Under these cir- 

 cumstances the organic and inorganic principles are so closely 

 united, that the latter may be said to acquire, by virtue of 

 this union, vital properties. Though unaltered, the inorganic 

 are discharged with the worn-out organic substances, and, 

 combined with fresh organic matter, are deposited in the 

 tissues in the process of regeneration. 



3. The organic principles which are naturally fluid may 

 be coagulated, but under no circumstances do they assume a 

 definite or crystalline form. We should be led to expect this 

 from the fact that they have no absolutely fixed composition. 

 When the liquids of this class are thus solidified, they are 

 not precipitated from a solution, but are made to assume 

 a new form, still retaining their water of composition. 

 When exposed to evaporation, whether they be fluid or semi- 

 solid, their water may be driven off, and they are said to be 

 desiccated. They can be made to assume their water of 

 composition again by simple contact, as they have in a high 

 degree the property of hygrometricity. Both these properties 

 are peculiar to organic substances. 



4. When exposed to a very elevated temperature, that 

 which has been considered by chemists as the organic sub- 

 stance proper is volatilized and driven off, leaving the inor- 

 ganic substances, which always enter into its composition. 



5. In their natural condition, the organic principles have 

 no very distinct odor ; but when exposed for some time to a 

 moderate heat, certain odorous or empyreumatic substances 

 are produced. This change is peculiar to organic matters, 

 and takes place in the process of cooking. When these ele- 

 ments are used as food, this process serves a useful purpose, 

 rendering them more agreeable to the taste, and facilitating 

 their digestion. 



6. One of the great distinctive properties of organic priu- 



