294 CHEMISTRY. 



the difference in their composition. All the four grand 

 elements carbon, hydrogen, oxygen, and nitrogen enter 

 into the structure of animals, but only the first three are 

 found in the structure of vegetables. The inquiry then 

 arises in what way the nitrogen is supplied to animals. 

 Nitrogen constitutes four fifths of the air which is so con- 

 stantly entering their lungs, and yet not a particle of it is 

 supplied to their bodies in this way. The blood in the 

 lungs receives oxygen from the air, but no nitrogen, as you 

 learned in 132. The nitrogen which is needed is supplied 

 through the agency of plants. For this purpose, though 

 there is none of this element in their structure, many of 

 them have it in their juices and fruits. It is especially 

 present in Indian corn, the grains, pease, beans, etc., so ex- 

 tensively used for food. In such cases the plant may be 

 said to gather up this element, and deposit it, not in its 

 own structure, for it is not wanted there, but in repositories, 

 where man and other animals can take it and appropriate 

 it to their use. There is no case in which the design of 

 the Creator is manifested in a more marked manner than 

 it is here. 



411. Definition of Organic Chemistry. Of the four ele- 

 ments C, H, O, and N" playing such a wonderful part in 

 the vegetable and animal kingdoms, the first of these, carbon, 

 is by far the most important, its presence being character- 

 istic of organic substances. Hence organic chemistry is 

 often defined as the Chemistry of Carbon Compounds. This 

 definition includes the simple carbon compounds, carbonic 

 oxide, carbonic anhydride, and others which we have just 

 studied under the head of mineral chemistry ; but it is im- 

 possible to draw any precise line of distinction, especially 

 since the same elements and laws of union are common to 

 the two divisions. Organic bodies are characterized by 

 great complexity and instability of the molecules. 



