198 RESPIRATION 



40. We have seen that the whole animal creation is con- 

 stantly taking oxygen from the atmosphere, and as constantly 

 adding to it vast volumes of gas, that is prejudicial to the 

 normal performance of animal respiration. How, then, does 

 the air retain, unchanged, its life-giving properties ? The 

 uniform purity of the air is secured by means of the vegetable 

 creation. Carbonic acid gas is the food of the plant, and 

 oxygen is its waste product. The leaves are its lungs, and 

 under the stimulus of sunlight a vegetable respiration is set in 

 motion, the effects of which are just the reverse of that of 

 animals. Thus nature purifies the air, and at the same time 

 builds up beautiful and useful worlds the life of each growing 

 out of the decay of the other. (Read Note 10.) 



41. 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. 



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



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



