A HISTORY OF SCIENCE 



If, on the other hand, a gas is subjected to pressure, 

 its molecules are crowded closer together, and the 

 length of their mean free path is thus lessened. Ulti- 

 mately, the pressure being sufficient, the molecules are 

 practically in continuous contact. Meantime the enor- 

 mously increased number of collisions has set the mole- 

 cules more and more actively vibrating, and the tem- 

 perature of the gas has increased, as, indeed, necessarily 

 results in accordance with the law of the conservation 

 of energy. No amount of pressure, therefore, can 

 suffice by itself to reduce the gas to a liquid state. It 

 is believed that even at the centre of the sun, where the 

 pressure is almost inconceivably great, all matter is to 

 be regarded as really gaseous, though the molecules 

 must be so packed together that the consistency is 

 probably more like that of a solid. 



If, however, coincidently with the application of 

 pressure, opportunity be given for the excess of heat 

 to be dissipated to a colder surrounding medium, the 

 molecules, giving off their excess of energy, become 

 relatively quiescent, and at a certain stage the gas be- 

 comes a liquid. The exact point at which this trans- 

 formation occurs, however, differs enormously for 

 different substances. In the case of water, for exam- 

 ple, it is a temperature more than four hundred de- 

 grees above zero, centigrade ; while for atmospheric air 

 it is one hundred and ninety-four degrees centigrade 

 below zero, or more than a hundred and fifty degrees 

 below the point at which mercury freezes. 



Be it high or low, the temperature above which any 

 substance is always a gas, regardless of pressure, is 

 called the critical temperature, or absolute boiling- 



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