352 THE AIR OF TOWNS. 



free oxygen is removed from the air by entering into combination with 

 the substances which it burns or consumes. 



A piece of charcoal is attached to an iron rod, which passes through 

 a metal plate (fig. 2). The charcoal is first heated until it begins to 

 glow, and is then brought into a glass jar containing oxygen. The 

 charcoal immediately glows with dazzling whitness by uniting with the 

 oxygen to form carbonic acid. 



I shall have very little more to say about these two gases, but shall 

 now direct your attention to another gas, carbonic acid, which is always 

 present in the air, usually in a minute quantity. Its presence may be 

 most readily shown by exposing to the air some clear limewater in a 

 glass basin, when the surface is soon coated with a white film of car- 

 bonate of lime. It is also a very heavy gas, as I can show you by the 

 following experiments: 



In fig. 3, a represents the vessel containing the clear lime-water, 

 which on standing becomes covered with a white film of carbonate of 

 lime; h represents the vessel containing the heavy gas, carbonic acid, 

 upon which the soap bubble floats. The apparatus figured at c is for 

 generating carbonic acid. It consists of two vessels, which are con- 

 nected by glass tubing. The larger vessel contains marble. By pour- 

 ing acid down the funnel a brisk effervescence occurs, carbonic acid 

 being evolved, which bubbles through the second vessel containing 

 water to remove impurities, and is then used for filling B with gas. 



A large glass beaker (fig. 4) is suspended at one arm of a balance and 

 carefully counterpoised. By slowly inverting another beaker contain- 

 ing carbonic acid above the open mouth of the suspended one, the 

 latter becomes filled with the heavy gas and descends. 



The following table gives the volumes of the different gases in pure 

 air in 100 volumes and also the total weight of these gases: 



Composition of the atmosphere. 



Oxygen 20.61 



Nitrogen 77. 95 



Carbonic acid 0. 03 



Aqueous vapor 1. 40 



Nitric acid j 



Ammonia [Traces. 



Ozone j 



Composition of the atmosphere in tons. 



Millions of tons. 



Oxygen 1,233,010,000 



Nitrogen 3,994,593,000 



Carbonic acid -5,287,000 



Aqueous vapor 54, 460, 000 



Where does carbonic acid gas come from"? From coal, charcoal, or 

 other fuel when it burns. (The jar in which the charcoal was previ- 

 ously burnt in oxygen was shaken with limewater, and by becoming 



