242 Domestic Science 



the total quantity produced during the period since 

 animals first came into existence and fire was dis- 

 covered must be very great. A corresponding amount 

 of oxygen must have been removed from the air in 

 order to combine with the carbon and thus form the 

 carbon dioxide. It would therefore seem probable 

 that the percentage of carbon dioxide in the air would 

 increase, and that of oxygen would diminish, in the 

 course of- ages. We believe, however, that these 

 percentages do not appreciably vary, and two factors 

 are concerned in this unchangeableness of the amounts 

 of the two gases to be found in the atmosphere. Firstly, 

 the carbon dioxide formed at any spot is rapidly diffused 

 through the surrounding air, and the total mass of the 

 Earth's atmosphere is so enormous that it would take 

 an exceedingly long time for the proportion of carbon 

 dioxide found in an ordinary sample of " fresh " air 

 to be altered to a sufficient extent for the change in 

 amount to be detected even by the most delicate 

 methods at our disposal. Secondly, the carbon dioxide 

 a waste product of the animal organism provides 

 food-material for plants. Their structure is such that 

 they are capable of absorbing carbon dioxide from the 

 air under suitable conditions, using the carbon to build 

 up their tissues, and returning oxygen to the air. 



EXPERIMENT 102. Fit up the apparatus shown in 

 the annexed figure. The flask should have a capacity 

 of about two litres and should be entirely filled with 

 ordinary tap-water. The common water-weeds are 

 the best plant material ; to place in the water as 

 shown. Put the whole apparatus in bright sun-light 

 with the test-tube, filled with water, fixed in the 

 position indicated in the figure. Bubbles of a gas will 

 be slowly disengaged from the surface of the leaves 



