THE COMPOSITION OF PLANTS AND ANIMALS 147 



the elements carbon, hydrogen and oxygen with nitrogen and 

 sometimes also sulphur and phosphorus in the important group 

 of the proteins. Some few other elements may also occur in 

 strictly organic combination, but these six elements are the con- 

 stituents of practically all of the organic compounds of both 

 plants and animals. 



The inorganic compounds present in plants and animals con- 

 sist mainly of the metallic elements, potassium (K), sodium 

 (Na), calcium (Ca), magnesium (Mg) and iron (Fe), in com- 

 bination, mostly as salts, with the nonmetallic elements chlorine 

 (CI), sulphur (S), phosphorus (P), nitrogen (N) and oxygen (O). 

 Aluminium (Al), arsenic (As) and some others may also be 

 present. It is not always possible to determine whether these 

 elements that we consider inorganic are present in the living 

 plant or animal in true inorganic combination as salts or whether 

 they really are part of a true organic compound containing also 

 carbon, hydrogen, oxygen, nitrogen, etc. We know, for ex- 

 ample, that in the blood, urine and gastric juice of animals we 

 find such compounds as sodium chloride, calcium phosphate 

 and hydrochloric acid, which are evidently present as such in 

 these liquids. On the other hand we believe that in the blood 

 the iron which is present is unquestionably in the form of a true 

 organic compound, a protein known as hcEmoglohin. Also in 

 chlorophyll, a constituent of green leaves, magnesium is present 

 as part of a true organic compound, and in milk the metallic 

 element calcium is present as part of one of the protein constit- 

 uents. Thus, if we should consider as organic any substance 

 present as part of the living material of a plant or animal, it is 

 probable that most of the inorganic elements mentioned should 

 be classed as in organic combination. 



Volatile and Nonvolatile Constituents. — The separation of 

 the constituents of plants and animals into organic and in- 

 organic may be easily accomplished in an approximate way by 

 simply burning the plant or animal substance in the air. When 

 thus burned in the air the organic constituents consisting in 

 general of the six elements previously mentioned are converted 



