THE COMPOSITION OF THE AIR 97 



core. This gaseous envelope, composed of these substances 

 surrounding the earth, we call the atmosphere. Some of these 

 gases are inert; that is, they do not readily form chemical 

 combinations with other, substances. Others have formed 

 extensive combinations, but they exist in such large quanti- 

 ties that they were not thereby exhausted. 



The Composition of the Air. Experiment 35. (Teacher's 

 Experiment.) Having rounded out a cavity in a small flat cork, 

 cover the cavity and surface around it with a thin layer of plaster 

 of Paris. After the plaster has set and become thoroughly dry, 

 float the cork on a dish of water with the cavity side up. Place 

 a piece of phosphorus as large as a pea in the 

 cavity and carefully light it. (Figure 48.) (Great 

 care must be taken in handling phosphorus, as it 

 ignites at a low temperature and burns with 

 great fierceness. It must always be cut and 

 handled under water.) 



As soon as the phosphorus is lighted, cover 

 it with a wide-mouthed bottle. Be sure that FIGURE 48 

 the mouth of the bottle is kept slightly under 

 water. The water will be found to rise in the bottle. The phos- 

 phorus soon ceases to burn. White fumes are formed, but these 

 soon clear up. A clear gas is left in the bottle, but this cannot 

 be air; for if it were, the phosphorus would have continued to 

 burn in it, since it burns in air. If it were not for this property 

 of not permitting phosphorus to burn, the gas left in the bottle 

 could not be distinguished by ordinary means from air. 



The gas fills more than three fourths of the bottle, so that more 

 than three fourths of the air is composed of a gas which does not 

 support combustion. This gas is called nitrogen. The other constitu- 

 ents of the air must also be transparent colorless gases, since the air 

 is transparent and colorless. The most important of these is called 

 oxygen. The phosphorus united with this and formed the white 

 fumes. These fumes dissolved in the water, leaving the nitrogen. 



Be careful to put the cork on which the phosphorus was burned 

 in a place where it cannot cause a fire. 



