x HEAT 153 



liquid more before the bubbles of vapour can escape at the 

 .surface than when the pressure is less. If we heat the liquid 

 more its temperature will get higher before there is any con- 

 version into vapour, and consequently its boiling point will be 

 higher when the pressure is greater. In finding the boiling 

 point of a liquid we must therefore know the pressure of the 

 atmosphere at that place and time. 



EXPT. 153. Get a well-fitting, sound cork to fit the neck of a 

 4 oz. round-bottomed flask. Boil water in the flask till all the 

 air is expelled. Remove the burner and cork up the flask as 

 rapidly as possible. Turn the flask over and pour cold water 

 on the up-turned bottom. Notice that as the pressure is dim- 

 inished by the condensation of the vapour the water boils again 

 at a lower temperature. 



Latent Heat of Fusion. It has been already pointed out that 

 when ice is melting the mixture of ice and water remains at the 

 same temperature as long as any ice is left, although the mixture 

 is being continuously heated. The heat which is thus absorbed, 

 and has no effect on the temperature of the mixture, is used up 

 in causing the change of state from solid to liquid, and since it 

 lies hidden, as it were, is referred to as the latent heat of fusion 

 or the latent heat of ivater. This quantity of heat can be defined 

 as being the number of units of heat which are required to 

 change the state of the unit mass of ice, converting it from 

 the solid to the liquid condition, without raising its tem- 

 perature. 



To melt one pound of ice requires as many heat units as are 

 necessary to raise a pound of water from O 3 to 80 C., or as much 

 heat as is wanted to raise 80 Ibs. of water through 1 C. In just 

 the same manner, before a pound of water can be changed into 

 a pound of ice we must take away from it just the same amount 

 of heat. Bearing this in mind, it is easy to understand why it 

 takes so many cold nights to cover a pond over with ice, and 

 why it takes such a long time, too, to completely melt the snow 

 in our roads even after the thaw has set in. 



Latent Heat of Vaporisation. When a liquid boils, as 

 we have seen, the thermometer remains stationary as long as 

 there is any liquid left. After boiling has commenced all the 

 heat is absorbed in changing the substance from the liquid to the 

 gaseous state. This heat also becomes "latent " and is referud 

 to as the latent heat of vaporisation or the latent heat of steam ; it 



