CLOUDS AND RIVERS, ICE AND GLACIERS. 153 



382. Expose water to a fire; it becomes hotter for a 

 time. It boils, and from that moment it ceases to get 

 hotter. After it has begun to boil, all the heat com- 

 municated by the fire is carried away by the steam, 

 though the steam itself is not the least fraction of a degree 

 hotter than the water. 



383. In fact, simply to liquefy ice a large quantity of 

 heat is necessary, and to vaporize water a still larger 

 quantity is necessary. And inasmuch as this heat does 

 not render the water warmer than the ice, nor the 

 steam warmer than the water, it was at one time sup- 

 posed to be hidden in the water and in the steam. 

 And it was therefore called latent heat. 



384. Let us ask how much heat must the sun expend 

 in order to convert a pound weight of the tropical ocean 

 into vapour ? This problem has been accurately solved 

 by experiment. It would require in round numbers 

 1,000 times the amount of heat necessary to raise one 

 pound of water one degree in temperature. 



385. But the quantity of heat which would raise the 

 temperature of a pound of water one degree would raise 

 the temperature of a pound of iron ten degrees. This 

 has been also proved by experiment. Hence to convert 

 one pound of the tropical ocean into vapour the sun 

 must expend 10,000 times as much heat as would raise 

 one pound of iron one degree in temperature. 



386. This quantity of heat would raise the tempera- 

 cure of 5 lbs. of iron 2,000 degrees, which is the fusing 

 point of cast iron ; at this temperature the metal would 



