x HEAT 151 



CHANGE OP STATE. 



Melting Point. The first effects on heating a solid, as has 

 been seen, are to raise its temperature and to cause an increase 

 in its volume. If we continue the process of heating, the solid 

 state gives place to a liquid condition, or the solid melts or fuses. 

 Common experience affords many examples of this, e.gr., when a 

 lump of lead is heated its temperature rises and it gets larger, 

 and as the heating is continued it is converted into a silvery 

 liquid. Wax, ice, and iron are other examples. But the tem- 

 perature at which the liquid state is assumed, i.e., the melting 

 point, is widely different in the case of different substances, as 

 the following table shows : 



Examples of Melting Points. 



Ice 0C. Tin 230 C. 



Beeswax 65 Lead 330 



Sulphur 115 Cast Iron . . . 1200 



EXPT. 150. Pound some pieces of clear ice and thrust a ther- 

 mometer into the powdered mass. Record the temperature 

 indicated by the thermometer. 



Put some of the ice into a beaker and pour in some water, 

 stir the mixture and again record the temperature. 



Place the beaker on a piece of wire gauze or in a sand bath 

 and warm gently. Notice the reading of the thermometer as 

 long as there is any ice unmelted. In all these cases the tem- 

 perature is the same, or the temperature of melting ice is 

 constant. 



EXFT. 151. Soften a piece of glass tubing in the flame of a 

 blowpipe and, removing it from the flame, draw it out until 

 the bore becomes very small. Cut off a piece of it and clip it 

 into some melted beeswax. In this way the fine tube becomes 

 filled with wax which soon solidifies. Tie this filled tube on 

 to a thermometer, near its bulb, and put the thermometer into 

 a beaker of water which has been placed over a burner in the 

 way shown later in the Chapter on Sulphur (Chapter XVII.) 

 Gradually heat the water and notice when the wax melts, and 

 at that instant read the thermometer. This reading will be 

 the melting point of the wax. 



Boiling Point. In explaining the nature of the process by 

 which liquids are heated, it was seen that the bottom layers of 



