CHEMICAL CHANGES IN PLANTS AND ANIMALS. 641 



duced at first, more complicated ones will be formed gradually by replacement 

 of more hydrogen, oxygen, or other atoms by other residues. 



The following equations, while not showing the various radicals and inter- 

 mediate compounds formed, may illustrate some of the results obtained by the 

 plant in forming organic compounds : 



C0 2 + H 2 == H 2 CO 3 



H 2 CO 3 O = H 2 C0 2 = Formic acid. 

 2C0 2 + H 2 = H 2 C 2 5 



H 2 C 2 O 5 O = H 2 C.,O 4 = Oxalic acid. 

 6C0 2 + 6H 2 = C 6 H 12 18 



C 6 H 12 O 18 12O = C 6 H 12 O 6 = Glucose. 

 10C0 2 + 8H 2 = C 10 H 16 28 



Ci H 16 O, 8 28O = C 10 H 16 = Oil of turpentine. 

 10C0 2 + 4H 2 + 2NH 3 =* C, H 14 O 24 N 2 



C 10 H U 24 N 2 24O = C 10 H U N 2 = Nicotine. 



The above formulas show that the formation of organic compounds in the 

 plant is always accompanied by the liberation of oxygen, and it may be stated, 

 as a general rule, that no organic substance (produced in nature) contains a 

 quantity of oxygen sufficient to convert all carbon into carbon dioxide and all 

 hydrogen into water, which fact also explains the combustibility of all organic 

 substances. 



Why it is that the living plant has the power of forming organic substances 

 in the manner above indicated, we know not, and we know very little even in 

 regard to the means by which the living cell accomplishes this formation, fcut 

 we do know that sunlight furnishes the kinetic energy necessary in the forma- 

 tion of complex substances from the simpler ones. This kinetic energy is 

 transformed into the potential energy, or chemical tension, of the new com- 

 pounds and of the liberated oxygen. 



Animal food. Those substances which when taken into the body 

 yield energy, build tissue, or prevent the consumption of tissue, 

 without injury to the organism, are called animal foods. The food 

 taken by animals is (beside water and a few of its mineral con- 

 stituents) all derived from vegetables, but it is taken from them 

 either directly or indirectly ; in the latter case it has been taken pre- 

 viously into and assimilated by other animals, as in case of food 

 taken in the form of meat, milk, eggs, etc. While some animals 

 (herbivora) feed upon vegetable, and some (carnivora) upon animal 

 food exclusively, others are capable of taking and assimilating either. 



The fact that animal food is derived from vegetable matter renders 

 it superfluous to state that the elements taking an active part in the 

 formation of either vegetable or animal matter are identical. Of the 

 total number of the elements, only 15 are found as necessary con- 

 stituents of the animal body. These elements are carbon, hydro- 

 gen, oxygen, nitrogen, sulphur, phosphorus, chlorine, iodine, fluorine, 

 silicon, calcium, magnesium, sodium, potassium, and iron. A few 



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