Minor Constituents of Air 247 



has disappeared as the result of the treatment with the 

 pyrogallic acid is that of the oxygen present. Compare 

 the volume-ratio between the nitrogen and the oxygen, 

 as thus obtained from water, with that in which they 

 exist in the air. 



Exercise for Student. 



Attempt to account for any difference between the two volume- 

 ratios mentioned above. 



We shall be able to make use of the observations 

 recorded in this experiment in later work dealing with 

 the nature of the two substances, air and water. 



167. That many metals, more especially iron, rust 

 when exposed to the action of water is a familiar 

 everyday experience. We will next try to show the 

 conditions under which rusting may or may not take 

 place. 



EXPERIMENT 106. Completely fill a jar with recently 

 boiled and cooled distilled water. Into this drop a 

 piece of clean bright iron. A new screw which has been 

 well washed with soap and water and thoroughly rinsed 

 will serve excellently. Allow the screw to remain in 

 the tightly stoppered jar for several days and examine 

 its appearance daily. 



Cover the bottom of a second jar with a layer of 

 small lumps of calcium chloride. Cork up the jar and 

 put it on one side for a day. At the end of that interval 

 hang a second screw similarly cleaned and then dried 

 in the jar by means of a piece of string kept in position 

 by the cork, in such a way that the screw does not touch 

 the chloride layer. As before, examine the appearance 

 of the screw daily for some days. 



Drop enough tap -water into a third jar to cover 

 the bottom to the depth of about one-tenth of an 



