SCIENCE OF COMMON THINGS. 



How water plants purify the water. Cause of animal beat. 



1579 Js there any difference between the respiration of plants and 

 animals f 



The process of respiration in plants is exactly the 

 reverse of that in animals. Animals absorb oxygen, and 

 give out carlo nic acid ; plants, on the contrary, absorb 

 carbonic acid, and return oxygen. 



15SO It is estimated that the population of London adds to the atmo- 

 sphere daily. 4, 500,000 pounds of carbonic acid: how is this immense quan- 

 tity of deleterious gas removed from the atmosphere f 



Principally through the agency of plants, which 

 absorb it. 



1581 Do water-plants purify and free water from carbonic acid in the 

 same manner that land-plants purify the atmosphere ? 



The respiration of fishes produces carbonic acid, and 

 anless this is removed from the water, animal life will 

 cease to exist in it. Water-plants absorb the carbonic 

 acid from the water, and restore the oxygen. 



1583 During bright weather, the leaves of water-plants, it will be no- 

 ticed, are covered with little bubbks : what are these bubbles 1 



Oxygen gas, liberated by the organs of the plant. 



1583 It is good policy, in fountains and reservoirs of water, to free 

 them wholly from the presence of vegetable and animal organisms ? 



It is not : they are both dependent on one another, 

 and the joint action of the two serves to keep the water 

 pure and wholesome. 



1584 What is the cause of animal heat? 



The oxygen of the atmosphere, received into the blood 

 in the lungs, and circulated throughout every part of 

 the animal body, acting upon the elements of the food, 

 is the chief source of animal heat. 



1585 Why does oxygen received into the blood produce heat ? 



Through the medium of the capillary vessels oxygen 

 absorbed from the atmosphere unites with carbon and 

 hydrogen. This union is a species of combustion, and 

 produces heat in the same manner as when oxygen 

 unites with fuel in an ordinary fire. 



1586 What are the capillary vessels f 



Minute bloodvessels or tubes as small as hairs run- 



