RESPIRATION. 141 



39. Nature's Provision for Purifying the Air. 



We have seen that carbonic acid is heavier than air, and 

 is poisonous. Why, then, does it not sink upon and over- 

 whelm mankind with a silent, invisible wave of death ? 

 Among the gases there is a more potent force than gravity, 

 which forever precludes such a tragedy. It is known as 

 the diffusive power of gases. It acts according to a defi- 

 nite law, and with a resistless energy compelling these 

 gases, when in contact, to mingle until they are thor- 

 oughly diffused. The added influence of the winds is 

 useful, by insuring more rapid changes in the air; air in 

 motion being perfectly wholesome. The rains also wash 

 the air. 



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

 stantly abstracting oxygen from the atmosphere, and as 

 constantly adding to it vast volumes of a gas injurious alike 

 to all, even in small quantities. How, then, does the air re- 

 tain, unchanged, its life-giving properties ? The constant 

 purity of the air is secured by means of the vegetable cre- 

 ation. Carbonic acid is the food of the plants, 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 the 

 function we have been considering. Thus nature purifies 

 the air, and at the same time builds up beautiful and use- 

 ful forms of life from elements of decay. 



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

 is observed. Marine animals consume oxygen and give off 

 carbonic acid; while marine plants consume carbonic acid 

 and liberate oxygen. Taking advantage of this fact, we 

 may so arrange aquaria with fishes and sea-plants, in their 

 proper combinations, so that each supplies the needs of the 

 other, and the water requires seldom to be renewed. This 



39. What can you state of the diffusive power of gases ? The added influence 

 of the winds ? 



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



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



