518 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



Through these agencies the tension of carbonic acid is kept hiffh in the Mood, 

 and its escape through the walls of the alveolar capillary is not unlike the 

 escape of gas on uncorking a bottle of carbonated water. 



After drinking a carbonated water, carbonic oxide may be detected dissolved 

 in the urine. 



Properties. — A colorless, odorless gas. It is poisonous, its accumulation at 

 first stimulating and afterwards paralyzing the nervous centres. It affects the 

 irritability — not, however, the conducting power — of the nerves. A solution 

 of carbonic oxide in water forms carbonic acid, H 2 C0 3 , and from this are derived 

 two series of salts, primary or acid salts, MHC0 3 , and secondary or neutral 

 salts, M 2 C0 3 . 



Metabolism of Carbon. — It will be remembered that there is a union of 

 chlorine and hydrogen on exposure to sunlight. In a similar manner the chloro- 

 phvll-containing leaf of the plant, through the medium of the energy of the sun's 

 ravs, brings the molecules of water and carbonic oxide derived from the air in 

 such a position with regard to each other that they unite to form sugar with the 

 elimination of oxygen (reaction on p. 501). This process is called synthesis — 

 the construction of a more complicated body from simpler ones. The active or 

 "kinetic" energy from the sun required to build up the compound is stored, 

 becoming " potential " energy in that compound, and is liberated again in 

 exactly the same quantity on the resolution of the substance into its original 

 constituents. So the amount of energy liberated in the decomposition of a 

 food in the body is exactly equal to the energy needed to build it up from 

 its excreted constituents, 1 and this liberated energy appears in the body as 

 heat, work, and electric currents. 



The plant has the power of converting sugar into starch and cellulose, and 

 likewise into fat. Further the sugar undoubtedly unites with certain nitrogen- 

 containing bodies, and the synthesis of proteids results. Plants containing this 

 mixture of food-stuffs become the sustaining basis of animal life. The animal 

 devours these substances and either adds them to his body, or burns them to 

 prevent destruction of his own substance: such are the objects of food. In 

 contradistinction to synthesis in plants, animal life is said to be characterized by 

 analysis, i. e., the resolution of a complicated substance into simpler ones. This 

 classification is not entirely accurate, many exceptions occurring on both sides; 

 for example, animals may convert sugar into fat, which is synthesis. The 

 animal discharge- its carbon partly as carbonic acid, and partly in the form 

 of inure complex organic compounds, such as urea and uric acid. Since these 

 latter after leaving the body eventually become oxidized, and the carbon 

 becomes completely changed to carbon dioxide, it follows that all animal carbon 

 i- finally restored to the air in the form of carbon dioxide. Thus is established 

 the revolution of the carbon atom, made possible by the energy of the sun, 

 between air, plants, animals, and back to air again. Burning coal, lime-kilns, 

 volcanoes, give carbonic acid to the air. Rain water receive- carbonic acid 

 from the atmosphere, from putrefying organic matter in the soil and from the 

 1 See Rubner, Zeitschrift fur Biologic, L893, lid. 30, S. 73. 



