LESSONS IN PHYSICS. 25 



ferrule of an old penholder ; then a little candle wicking and an 

 extinguisher, which may be made of cork, completes the lamp. 

 Take a piece of glass tubing, from eight to ten inches long, soften 

 it near the middle in the alcohol flame and bend it to an angle of 

 about 60 degrees, and, when cold, fit it in one of the perforated 

 corks. Fill a test tube with water and force in the cork, carry ing 

 the glass tube so that the water rises in the tube through the 

 cork ; then wipe dry, place the lower end of the test tube in the 

 alcohol flame, and notice the water in the smaller tube; with- 

 draw the heat and again observe the effect. The experiment 

 may be more interesting if colored water is used. If a vessel be 

 filled with cold water and heat applied, the water will expand and 

 overflow before it is hot enough to boil; this is a common oc- 

 currence in the cook room. Empty the water from the test tube, 

 used in showing the expansion of water, and when dry, close it 

 with the cork carrying the delivery tube ; then dip the free end 

 of the tube under water and apply the flame to the test tube. 

 The bubbles formed indicate that the air in the tube is expanding; 

 that some of it has been forced out. Remove the flame, keeping 

 the end of the delivery tube under the water, and soon the water 

 will be seen rising in the carrying tube; showing that the air 

 is contracting. These and other experiments indicate that heat 

 causes solids, liquids and gases to expand, and that when heat is 

 removed they contract, the attractive forces drawing the mole- 

 cules closer together. 



The forces of heat and cohesive attraction, acting upon the 

 molecules of bodies, are called molecular forces, and from the 

 direction of their action are called attractive or repellant forces. 

 They are constantly struggling between the molecules of bodies, 

 and as the one or the other prevails the body is hot or cold, 

 is a solid, a liquid, or a gas. In a solid body the attractive 

 forces prevail, and its molecules are firmly bound together, so 

 that solid bodies tend to retain the form given them by nature 

 or art. In a liquid body the attractive and repellant forces are 

 nearly equal, and the molecules are free to move easily among 

 themselves, so that the form of liquids depends on the form of 



