A HISTORY OF SCIENCE 



when cooled to 182, support a weight of two 

 pounds, and would vibrate like a steel spring so long 

 as it was cool. A bell of fusible metal has a distinct 

 metallic ring at this low temperature; and balls of 

 iron, tin, lead, or ivory cooled to 182 and dropped 

 from a height, "in all cases have the rebound greatly 

 increased. The flattened surface of the lead is only 

 one-third what it would be at ordinary temperature." 

 "These conditions are due solely to the cooling, and 

 persist only while the low temperature lasts." 



If this increased strength and hardness of a con- 

 tracted metal are what one would expect on molecular 

 principles, the decreased chemical activity at low tem- 

 peratures is no less natural-seeming, when one reflects 

 how generally chemical phenomena are facilitated by 

 the application of heat.' In point of fact, it has been 

 found that at the temperature of liquid hydrogen 

 practically all chemical activity is abolished, the un- 

 ruly fluorine making the only exception. The ex- 

 planation hinges on the fact that every atom, of any 

 kind, has power to unite with only a limited number 

 of other atoms. When the " affinities" of an atom are 

 satisfied, no more atoms can enter into the union un- 

 less some atoms already there be displaced. Such 

 displacement takes place constantly, under ordinary 

 conditions of temperature, because the vibrating atoms 

 tend to throw themselves apart, and other atoms may 

 spring in to take the places just vacated such inter- 

 change, in fact, constituting the essence of chemical 

 activity. But when the temperature is reduced the 

 heat-vibration becomes insufficient to throw the atoms 

 apart, hence any unions they chance to have made are 



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