Air 



215 



portion remaining imconsumed bears to the latter a 

 ratio of about 4 to 1. 



Exercise for Student. 



Detect other slight sources of possible error in the relative 

 volumes of air and of nitrogen thus obtained. 



145. The part of the air which supports burning 

 is called oxygen, while the other portion is termed 

 nitrogen 1 , and we have learnt that air is made up of 

 about 4 volumes of nitrogen with 1 volume of oxygen. 

 A second determination of this volume-ratio may be 

 made by the method of 



EXPERIMENT 81. Put about an ounce of clean iron 

 filings on a piece of muslin, and make the latter into 

 a bag by bringing the corners together and tying them 

 with thread. Attach this bag of iron filings to one end 

 of a length of stout copper wire, dip the bag in water 

 and arrange the apparatus 

 shown in Fig. 68, where M 

 is the bag, J a graduated 

 gas- jar similar to that used 

 in the last experiment, and 

 T an earthenware trough. 



Read off the volume of 

 enclosed air. 



Allow the apparatus to 

 stand for a week. Again 

 read off the volume of the 

 enclosed gas, and test its 

 nature as in Experiment 80. Fig. 68. 



Then remove the bag and examine the appearance 

 of the iron filings, comparing it with that before they 

 were inserted in the jar. 



1 See p. 222, 150. 



