T86 MELTING AND FREEZING. 



On a small scale this opposite flow of air may be demonstrated 

 by opening a few centimetres a door which leads from a warm room 

 into a cooler room or a passage, and holding a lighted candle in the 

 opening. The flame when held above, near the top of the door, is 

 blown/rom the room ; when placed below, near the floor, it is blown 

 into it ; when held midway between top and bottom it is blown 

 neither way and burns pretty steadily. 



55. Melting and Freezing. Heat diminishes the co- 

 hesion between the particles of a body, and increases 

 their expansive force. This is proved by the increase 

 in volume and the diminution of rigidity in solids which 

 are heated ; for example, red-hot iron is much softer 

 than cold iron. 



In solids the cohesion exceeds by far the expansive 

 force of the molecules (see page 18) ; but if by heating 

 a solid the cohesion is more and more diminished, 

 while the expansive force is continually increased, a 

 point will be reached when the cohesion exceeds the 

 expansive force only by a small amount, so that at a 

 certain point of temperature the solid body becomes 

 liquid ; it is then said to ' melt.' 0n the contrary, if a 

 liquid, by being cooled, becomes solid, it ' freezes ' or 

 * solidifies.' The temperatures at which solids melt or 

 liquids solidify, are very different for different sub- 

 stances, but each substance melts at a fixed tempera- 

 ture, which is the same as that at which the same sub- 

 stance in the liquid state freezes. These temperatures 

 are, respectively, called the ' melting point ' or ' point 

 of liquefaction,' and the ' freezing point ' or ' point of 

 solidification ' of the substance. For example, the melt- 

 ing point of ice is 0, and this is also the freezing point 

 of water. When ice which is colder than is brought 

 into a warm room, its temperature rises to (see page 



