A HISTORY OF SCIENCE 



the bulk of the gas is such that at this pressure it 

 occupies a cubical space six inches on a side something 

 like the bulk of a child's toy balloon, let us say. Then 

 the total outward pressure which that tiny bulk of gas 

 exerts, in its desperate molecular struggle, is little 

 less than five thousand tons. It would support an 

 enormous building without budging a hair's-breadth. 

 If the building weighed less than five thousand tons 

 it would be lifted by the gas ; if much less it would be 

 thrown high into the air as the gas expanded. It gives 

 one a new sense of the power of numbers to feel that 

 infinitesimal atoms, merely by vibrating in unison, 

 could accomplish such a result. 



But now suppose our portion of gas, instead of being 

 placed under our hypothetical building, is plunged into 

 a cold medium, which will permit its heat- vibrations 

 to exhaust themselves without being correspondingly 

 restored. Then, presently, the temperature is lowered 

 below the critical point, and, presto ! the mad struggle 

 ceases, the atoms lie amicably together, and the gas 

 has become a liquid. What a transformed thing it is 

 now. Instead of pressing out with that enormous force, 

 it has voluntarily contracted as the five thousand tons 

 pressure could not make it do; and it lies there now, 

 limpid and harmless -seeming, in the receptacle, for all 

 the world like so much water. 



And, indeed, the comparison with water is more than 

 superficial, for in a cup of water also there are wonder- 

 ful potentialities, as every steam-engine attests. But 

 an enormous difference, not in principle but in prac- 

 tical applications, exists in the fact that the potentiali- 

 ties of the water cannot be utilized until relatively 



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