PHYSICAL PARADOXES. 



much heat in melting itself that it reaches 

 thirty-two degrees of frost. 



When Fahrenheit first made this experi- 

 ment (with sal-ammoniac) and discovered what 

 intense cold it produced, he thought he had 

 reached the lowest temperature obtainable. 

 This is why that particular temperature was 

 chosen as the zero of the Fahrenheit scale ; 

 and so, instead of freezing point being marked 

 o, as on the Centigrade scale, and boiling point 

 180, we have freezing 32, and boiling 212. 

 We know now how great a mistake this was, 

 since in liquid air we have 344 degrees of frost, 

 while liquid and solid hydrogen are colder still. 



Still, thirty-two degrees of frost constitute 

 very severe cold. It is for this reason that 

 throwing salt upon the snow is an unsatisfactory 

 way of getting rid of it from the streets. It 

 makes the snow meltable, no doubt, at a very 

 low temperature, so that it disappears more 

 quickly by melting itself. But in doing so it 

 uses up so much of its own heat that it becomes 

 excessively cold for the feet of passengers and 

 horses. For we thus get its melting associated 

 not with heating but with growing colder. 



2. Ice melted without Heat or Cold. 



There is yet a third way of melting ice : 

 simply by squeezing it. If we were strong 

 enough, we could take a lump of ice in the 



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