248 HEAT. 



resting upon the surface, forms an effectual screen from 

 the cold winds to the water below. 



SECTION III. 



LATENT HEAT. 



If a vessel of snow, which has been cooled down to 

 several degrees below freezing by exposure to the se- 

 vere cold of winter, be placed over a steady fire with a 

 thermometer in the snow, the mercury will rise by the 

 increasing heat of the snow until it reaches the freez- 

 ing point. At this moment it will stop rising, and the 

 snow will begin to melt ; and although the heat is all 

 the time passing rapidly into the snow, the thermom- 

 eter will remain perfectly stationary till it is all con- 

 verted to water. The heat that goes to melt the snow 

 does not make it any hotter ; in other words, it becomes 

 latent (the Latin word for hidden), so as neither to af- 

 fect the sensation of the hand or to raise the thermom- 

 eter. Now it has been found that the time required 

 to melt the snow is sufficient to heat the same quantity 

 of water, placed over the same fire, up to 172 degrees, 

 or 140 degrees above freezing; that is, 140 degrees 

 have become latent, or hidden, in melting the snow. 



This same amount of heat may be given out again 

 by placing the vessel of water out of doors to freeze. 

 A thermometer will show that the water is growing 

 colder by the escape of the heat, till freezing commen- 

 ces. After this it still continues to pass off, but the 

 water becomes no colder till all is frozen, as it was 

 only the latent heat of the water that was escaping. 



A simple and familiar experiment exhibits the same 



