58 LABORATORY LESSONS IN GENERAL SCIENCE ' 



(b) why the downward pull of gravity on the water in the 

 long arm is greater than in the short arm ; (c) why water 

 in the tube flows out of the long arm. 



Repeat the experiment, but have the outer arm of the 

 siphon dip into water that fills another tumbler. Raise 

 and lower first one of the tumblers and then the other, 

 causing the water to flow back and forth. 



Where should measurements be taken to determine the 

 length (a) of the short arm of the siphon; (6) of the long 

 arm? 



7. From the lesson on Respiration recall and restate 

 how the capacity of the chest cavity is increased and how 

 it is diminished in natural breathing. Explain why air 

 goes into the lungs, and why it goes out, making use of such 

 drawings of the chest cavity as may be helpful. 



CAUSES OF CHANGE IN ATMOSPHERIC PRESSURE 



1. Close a test tube with a rubber stopper, through which 

 passes a glass tube. Invert the tube and hold the end of 

 the glass tube under water. Heat the test tube, and ob- 

 serve what occurs at the mouth of the glass tube. Explain 

 the behavior of the air of the tube. What is true of the 

 density of the remaining air? 



Allow the tube to cool with the glass tube still under 

 water, and account for what occurs. What is true of the 

 density of air as it cools? 



2. The weight of a cubic foot of air and of the vapor it 

 contains in grains, with the barometer at 29 inches, and 

 with the temperature in Fahrenheit degrees, is approximately 

 as follows : 



