208 RESPIRATION 



41. Oxidation. When a substance capable of undergoing 

 chemical changes unites with oxygen, it is said to oxidize. 

 This process of oxidation is always accompanied by the pro- 

 duction of heat. As an example, when coal is burned, its 

 carbon and other component parts unite with oxygen and form 

 new chemical compounds, thus producing a great deal of heat. 

 The chemical processes going on during the burning of coal 

 set free a certain amount of energy, which is utilized in the 

 production of steam and other 'kinds of power. Thus, in an 

 analogous manner, do the chemical processes going on con- 

 stantly in our body by the aid of the oxygen produce heat and 

 energy. (Head Note 15.) 



42. In the sea, as in the air, the same circle of changes is 

 observed. Marine animals consume oxygen and give off car- 

 bonic acid gas, while marine plants consume carbonic acid gas, 



10. Plants and the Air. " Though the air is dependent for the renewal 

 of its oxygen on the action -of the green leaves of plants, it must not be 

 forgotten that it is only in the presence and under the stimulus of light 

 that these organisms decompose carbonic acid gas. All plants, irrespec- 

 tive of their kind or nature, absorb oxygen and exhale carbonic acid gas 

 in the dark. The quantity of noxious gas thus eliminated is, however, 

 exceedingly small when compared with the oxygen thrown out during the 

 day. Aside from the highly deleterious action that plants may exert on 

 the atmosphere of a sleeping-room, by increasing the proportion of car- 

 bonic acid gas during the night, there is another and more important 

 objection to be urged 'against their presence in such apartments. Like 

 animals, they exhale peculiar volatile organic principles, which^in many 

 instances render the air unfit for the purposes of respiration. Even in 

 the days of Andronicus this fact was recognized, for he says, in speaking 

 of Arabia Felix, that. 'by reason of myrrh, frankincense, and hot spices 

 there growing, the air was so obnoxious to their brains, that the very 

 inhabitants at some times cannot avoid its influence.' What the influence 

 on the brains of the inhabitants may have been does not at present interest 

 us ; we have only quoted the statement to show that long ago the emana- 

 tions from plants were regarded as having an influence on the condition 

 of the air ; and, in view of our present ignorance, it would be wise to banish 

 them from our sleeping apartments, at least until we are better informed 

 regarding their true properties." Draper on Poisoned Air. 



41. How is the constant purity of the air secured ? Explain the process. 



42. What process occurs in the sea ? How is the fact illustrated ? 



