MATTER AND ENERGY 



Broken solids cannot be mended by simply pushing 

 the pieces together, because the molecules cannot be 

 forced near enough to each other. Some substances, 

 such as iron, may be heated until soft, however, and 

 then the broken ends may be pounded until they unite. 

 This process is called welding. 



15. Adhesion. Adhesion is the force which holds the 

 molecules of one body to those of another. Only a few sub- 

 stances have this property, and even they will not adhere 

 to many others. No paste, glue, or cement will stick to 



A everything; each 



is made for cer- 

 tain substances. 



Experiment 10. 



Balance a piece of 

 glass, a (Fig. 6), with 

 weights on the pan b. 

 Place a vessel beneath 

 a, and pour water 

 into the vessel till its 

 surface just touches 

 the piece of glass. Add more weights to the pan b until the glass 

 a is lifted from the water. Why do you have to add more weights ? 

 How much more do you add ? What force are you now measuring ? 



16. Hardness. A hard substance is one in which 

 the molecules resist any change of position. This prop- 

 erty of course applies only to solids, for the particles 

 of liquids and gases move about freely. 



Experiment 11. Using pieces of wood, glass, iron, copper, 

 lead, soap, and quartz, try to scratch each with the others. 

 Which scratches all of them, and which scratches none ? Arrange 

 them in a scale according to hardness. 



FIG. 6 



