COMPOSITION OF AIR 111 



these gases are inert and do not readily combine with 

 other substances. Others have formed extensive com- 

 binations, but they exist in such large quantities that they 

 were not thereby exhausted. 



61. The Composition of the Air. Experiment 57. (To be 

 performed only by the teacher.) 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. (Great care 

 must be taken in handling phosphorus as it ignites 

 at a low temperature and burns with great fierce- 

 ness. It must always be cut and handled under Fig. 47. 

 water.) 



As soon as the phosphorus is lighted, cover it with a wide-mouthed 

 bottle. Be sure that 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 sup- 

 port combustion. This gas is called nitrogen. 'The other constituents 

 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. 



Although the air appears like a simple gas and was so con- 

 sidered until the end of the eighteenth century it has been 

 shown to be composed of several different colorless gases. 

 One of these, oxygen, supports combustion ; another, 



