962 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



Detection. If expired air, or air from a bag enclosing any part of the skin, 

 be passed through a solution of calcium or barium hydrate, a precipitate of 

 white insoluble carbonate will be thrown down. 



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- 

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

 rays, 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. 945). 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 expires its carbon partly as carbonic acid, and partly in the form of 

 more 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 

 is 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 receives carbonic acid 

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

 roots of trees, and ultimately much of this combines with mineral matter, or 

 contributes to form shells in marine life. 



SILICON, Si = 28. 



Silicon is found in the ash of plants, and in traces in the cells and tissues of 



animals, being a constant constituent of hen's eggs. It appears in traces in the 



human urine, and in considerable quantity in herbivorous urine. It is especially 



present in hair and feathers. It does not seem to be of great importance to the 



1 See Kubner, Zeitschri/t fur Biologic, 1893, Bd. 30, p. 73. 



