62 EXPERIMENTAL GENERAL SCIENCE 



given back to the air. The British Islands, though in the lati- 

 tude of Labrador, have a much milder climate because of the 

 Gulf Stream which flows past their shores. One reason why 

 water puts out fires is because the heat needed to warm it 

 reduces the temperature below the point at which the fire will 

 burn. 



59. Latent Heat. Although a calorie of heat is said to raise 

 a gram of water one degree Centigrade, when the boiling point 

 is reached this statement needs qualifying. We then discover 

 that the addition of another calorie does not turn the boiling 

 water to steam, nor does it raise its temperature one degree; 

 in fact, it is not until 536 more calories are added that the water 

 will turn to steam, and then the steam is the same temperature 

 as the boiling water. In other words, it requires 536 times as 

 much heat to turn a gram of water to steam as it does to raise 

 its temperature one degree. Five hundred and thirty-six cal- 

 ories seems thus to have disappeared or become latent during 

 the change of state. When we recall the effect ,of heat on the 

 motion of molecules, however, we realize that the heat energy 

 has not really disappeared but is employed in holding the mole- 

 cules farther apart. That this is true is shown by the fact 

 that when the gas (steam) contracts and turns back to a liquid 

 again, the 536 calories reappear as heat. A similar state of 

 affairs exists at the point where water freezes. When a gram 

 of water turns to ice, it gives off 80 calories, though the ice is 

 then no colder than the water from which it was made. When 

 the ice is melted, however, 80 calories of heat are required to 

 be put into it to effect the change of state, and the water still 

 has the temperature of zero; that is, 80 calories of heat have 

 become latent in the process. In a certain sense, therefore, 

 melting may be said to be a cooling process and freezing a 

 warming process since in melting each gram of water absorbs 

 80 calories and in freezing it gives off this heat. All substances 

 act like water with reference to heat when a change in state 



