VOLATILE LIQUIDS. 



When, therefore, we say that ice is cold we 

 merely mean that it has less heat than our 

 bodies or most surrounding objects, not that it 

 has no heat. In comparison with some things 

 it has a great deal of heat. In comparison 

 with liquid air ice has as much heat as red-hot 

 iron has in comparison with water. Conse- 

 quently, ice can put liquid air into the same 

 spheroidal condition as we saw that water is 

 put into by red-hot metal. Until it has lost a 

 good deal of its heat and got nearer the tem- 

 perature of the liquid air itself, ice is too hot to 

 make it boil ! 



At first the excessive heat of the ice (exces- 

 sive in comparison with the extremely cold 

 liquid air) keeps creating a cushion of vaporised 

 air-gas between itself and the liquid, so that 

 heat cannot pass directly by conduction, but 

 only slowly by radiation. 



But when the ice has been slowly giving up 

 heat in this way, it gets at last cooled down to a 

 temperature so much nearer that of the liquid 

 that the two can come into contact. Then heat 

 can pass more freely from the ice to the liquid, 

 and instead of merely rapid vaporisation we get 

 true and actual boiling. The ice has at length 

 got cold enough to make the liquid air boil ! 



After the boiling has gone on for some time, 

 and the ice has parted with more and more 

 of its heat, it becomes finally as cold as the 

 liquid air itself. Then, of course, no heat can 



173 



