INTRODUCTORY EXERCISES 21 



(Phosphorus was used instead of sulphur or charcoal in this 

 exercise because its oxide is a soHd which settles and dis- 

 solves in the water.) Has the water risen in the jar? 

 What part, by volume, of the jar does it occupy? Since 

 the phosphorus has used up all the oxygen in burning, about 

 what part of air must be oxygen? 



B. Fill the test tube with gas from the bell jar in the 

 manner described in Ex. II, B. Examine this gas. What 

 is its color? odor? taste? Place a lighted match in it. 

 What happens? Explain. (This gas is called nitrogen}) 

 Of what advantage is the presence of nitrogen in the air? 

 Why is a good draught necessary to make a fire burn freely? 

 If the body needs to take in oxygen constantly why can 

 we not live in a sealed room? 



X. — Composition of Water (Optional). 



Apparatus. — Electrolysis apparatus,^ sulphuric acid, four dry 

 cells, splinters of wood, test tubes, pneumatic trough or other dish 

 of water, glass and rubber connecting tubing. 



Directions. — Open the two stopcocks and fill the ap- 

 paratus with water containing 5% of sulphuric acid. When 

 the tubes are full and all air driven out, close the cocks ; ar- 

 range the four dry cells in series (positive pole of one con- 

 nected with negative pole of the next, and so on) ; and 

 connect the positive and negative poles of the series with the 



^Other gases (carbonic acid gas, argon, water vapor) are present in very- 

 small proportions. 



^ For the electrolysis apparatus shown on p. 22 may be substituted simpler 

 forms with nearly as good results. Simple forms are shown in Clark and 

 Dennis's *^ Elementary Chemistry,^^ page 33; and in Remsen\5 '' Chemistry, 

 Briefer Course.^' 



