PHENOMENA OF LIFE 21 



immediately kill protoplasm. " Hence ideas upon the chemistry 

 of the living object can be obtained only by deductions from 

 chemical discoveries in the dead object" (17, p. 102). Never- 

 theless, we have gained a considerable amount of knowledge as to 

 the chemical nature of protoplasm. Ninety-seven per cent of 

 the organic and inorganic constituents of protoplasm are made 

 up of four elements, occurring in the following proportions: 



Oxygen 65 per cent 



Carbon 18.5 per cent 



Hydrogen n per cent 



Nitrogen 2.5 per cent 



These and other less important elements form certain rather 

 definite groups of compounds, which we will now consider in some 

 detail. 



Water is the most important of the inorganic constituents; 

 it comprises more than 50 per cent of the weight of most 

 animals, and in some marine forms reaches as high as 99 per 

 cent. It seems remarkable that man is able to exist when 59 

 per cent of his body is made up of water. It not only occurs in 

 combinations, but also acts as a solvent for various substances 

 that are found in protoplasm. 



Aside from water the most important inorganic substances are 

 various salts and dissolved gases. Of the salts the chlorides, 

 carbonates, and phosphates of the commoner alkali and alkali- 

 earth metals predominate. The principal gases are carbon di- 

 oxide and oxygen. 



The organic compounds found in protoplasm are divided into 

 three general classes: proteids, carbohydrates, and fats. "'Of 

 these only the proteids and their derivatives have been demon- 

 strated with certainty as common to all cells; hence, they must 

 be set apart among the organic constituents of living matter as 

 the essential or general substances, in contrast to all special 

 substances " (17, p. 103). 



Proteids are always composed of the five elements, carbon, 



