58 EXPERIMENTAL GENERAL SCIENCE 



wires which have practically the same rate of expansion as 

 the glass bulbs themselves. Platinum, though very expen- 

 sive, is the most suitable metal for this purpose since it ex- 

 pands and contracts at nearly the same rate as glass. Other 

 metals would contract and let air into the bulb or expand and 

 break it. In pouring hot liquids into glass, as in canning, the 

 inside of the jar often expands so rapidly that the jar is cracked 

 before the outside can become heated and expand to match it. 

 For a similar reason a drop of cold water splashed on a hot 

 glass may break it by causing the part to cool and contract 

 too suddenly. Crucibles made of quartz expand and contract 

 very little with changes of temperature and when heated red 

 hot may be plunged into water without being broken. 



Practical Exercises 



1. Heat the ball of a ring-and-ball set for a short time and try to pass 

 it through the ring. Explain the effect noted. 



2. Fill a florence flask with water and cork with a one-hole stopper 

 through which passes a close-fitting glass tube. Put the flask on a piece 

 of wire gauze over a bunsen burner and heat. Explain the movement 

 of the water in the tube. . 



3. Empty out the water from the flask used in the preceding experi- 

 ment and stop up the glass tube with a drop of any liquid. Warm the 

 flask by holding the hand upon it. Explain the results noted. 



4. Hold the mouth of a tall cylinder or large bottle at some distance 

 above a flame until the air within has been thoroughly warmed. Then 

 quickly place the vessel, mouth down, in a dish of water. Explain 

 how the cooling of the air in the vessel makes more room for water in it. 



6. Could one make a thermometer of water? 



6. When the sun shines on a tall chimney or monument, will it bend 

 toward or away from the sun? Why? 



