THREE FORMS OF WATER. LATENT HEAT. 35 



It is indispensable to the nourishment both of plants and 

 of animals ; and it dissolves much of the other food with 

 which plants are nourished. 



106. At the usual temperature of the greater part of 

 the year, water is a transparent liquid, which, when pure, 

 has neither color, taste, nor smell. But while water is 

 the great solvent of vegetable food, it is itself dissolved 

 by heat, a still more powerful solvent. 



107. Water is found in the three forms or conditions 

 of ice, water, and vapor, according to the amount of heat 

 with which it is combined. 



(1.) With little or no heat, it is solid Ice or snow. If 

 extremely cold ice be placed in a kettle over a fire, it will 

 be found, by observing a thermometer with its bulb 

 placed within it, to rise gradually until it reaches 32. 

 It then begins to thaw or turn into water, and if a steady 

 fire be kept up, under the kettle, it continues to thaw 

 until all the ice becomes water. During all this time, 

 though heat from the fire is constantly entering it, 

 through the kettle, it continues of the same temperature, 

 just at 32. 



What has become of the heat ? It has been used up 

 in dissolving the ice and turning it into water. It has 

 not rendered the water warmer ; it is hidden or latent in 

 the water ; and is called the Latent Heat of the water. 

 Ice has been changed by combining with heat, into 



(2.) Water. If, now, the same steady fire be continued 

 under the kettle, the temperature of the water gradually 

 rises to the boiling point, 212, and then begins to boil. 

 With the same steady fire, the water will entirely boil 

 away, or evaporate, in a certain space of time. And it will 

 be found that it takes more than five times as much heat 

 to boil the water all away, as it had taken to raise it 180, 



4* 



