12 LABORATORY EXERCI8S8. 



the cylindrical graduate. What is the 

 volume of the water, or in other words 

 how much air was in the jar at first? 

 3. The water measured in i above represents 

 the volume of the oxygen and the water 

 in 2 the whole volume of air in the jar. 

 About what fractional part of air is oxy- 

 gen? What per cent, of oxygen is there- 

 fore found in air? What per cent, is 

 nitrogen? 



4. STUDY OF ATMOSPHERIC PRESSURE. 



Materials: Tumbler of water, piece of paper, apparatus for 

 preparing oxygen, apparatus for preparing nitrogen. (See Figs. 1 

 and 2.) 



A. Fill a tumbler nearly full of water and cover it 



with a piece of paper, pressing it down firmly 

 upon the glass. Carefully invert the tumbler 

 and then remove the hand from beneath the 

 paper. 



a. What do you observe? 



b. Slowly turn the glass until the paper 



stands vertically. Describe result. 



c. Turn the glass upright. Describe the 



appearance of the paper. 



d. (The air exerts in all directions at the 



level of the sea a pressure equal to 

 about fifteen pounds to the square 

 inch.) Keeping in mind this fact 

 explain the preceding experiments. 



B. In the preparation of oxygen explain why the 



