Chap. I] EXPLANATIONS AND DEFINITIONS 5 



solid which has iron and oxygen in it, the iron having combined 

 with some of the oxygen from the air. The iron and the iron rust 

 are evidently different in composition. 



ELEMENTS FOUND IN THE BODY 



The elements found in the body are : 



form 97 per cent 

 of total 

 weight of body. 



These elements are not, of course, found uncombined in the 

 body, but rather combined, usually in the form of rather complex 

 compounds. Protoplasm, for instance, is a compound of carbon, 

 hydrogen, nitrogen, oxygen, and phosphorus. 



ORGANIC AND INORGANIC COMPOUNDS 



The distinction between organic and inorganic compounds 

 dates back to an early period, when there was a belief that cer- 

 tain compounds of carbon found in living organisms could only 

 be built up through the agency of a vital force possessed by the 

 organism, which prevented their being synthesized in the chemical 

 laboratory. In distinguishing such they were spoken of as organic 

 compounds. However, when urea, one of these substances, was 



