ARTIFICIAL COLD 85 



96. Cold by Melting. The change from a solid to a 

 liquid state requires heat. If it can be performed by 

 some means other than directly applying heat, the sub- 

 stance will take in the necessary heat from wherever it 

 can be had. For example, salt causes ice to melt, but 

 the melting ice must have heat in order to liquefy; 

 thus, if the ice and salt be put into an ice-cream freezer, 

 the heat will be taken from the cream, causing it to 

 become solid. 



Experiment 76. Put a tablespoonful each of sal ammoniac 

 and ammonium nitrate (solid salts) into a tumbler of water. At 

 once stir the whole with a small test tube containing water 

 (Fig. 63). The solid salts dissolve (becoming liquid) 

 very fast. Does this process require heat? From what 

 is this heat taken? With what result? 



97. Cold by Vaporizing. In the 

 same way, liquids in turning to gases 

 take heat from substances around 

 them. 



Experiment 77. Pour a small amount 

 of alcohol on the hand, allowing it to 

 evaporate. Does it feel cold ? Blow it, to 



make it vaporize faster. Does it feel colder ? Try naphtha, ether, 

 or chloroform in the same way. Do bottles of these liquids seem 

 cool to the touch ? 



The making of artificial ice depends upon this princi- 

 ple. Liquid ammonia can be kept in its liquid state 

 only under great pressure ; as soon as the pressure is 

 removed, the ammonia vaporizes rapidly, requiring much 

 heat. This is done near boxes of water, so that the heat 

 is taken from the water, freezing it. 



