CHAPTER IV 

 CARBONIC ACID 



TWO chemical individuals stand alone in 

 importance for the great biological 

 cycle upon the earth. The one is water, the 

 other carbon dioxide. The one, for reasons 

 which we have just reviewed, is the most 

 familiar of all the varieties of matter. The 

 other rarely is seen except by chance, and with- 

 out scientific research never could have been 

 known for what it is in value to living things. 

 Yet these two simple substances are the com- 

 mon source of every one of the complicated 

 substances which are produced by living 

 beings, and they are the common end prod- 

 ucts of the wearing away of all the constitu- 

 ents of protoplasm, and of the destruction of 

 those materials which yield energy to the 

 body. 1 



1 A man weighing 60-70 kilograms excretes daily the fol- 

 lowing quantities of material : — 



Water 2500-3500 grams 



Carbon dioxide .... 750-900 grams 



All other substances . . 60-125 grams 



133 



