TI1E ETHER AND PONDERABLE MATTER 



atmospheric temperature in tropical and in arctic regions 

 there is often a variation of more than one hundred de- 

 grees; were the temperature reduced another hundred, 

 the point would be reached at which oxygen gas becomes 

 a vapor, and under increased pressure would be a liquid. 

 Thirty-seven degrees more would bring us to the critical 

 temperature of nitrogen. 



Nor is this a mere theoretical assumption ; it is a 

 determination of experimental science, quite indepen- 

 dent of theory. The physicist in the laboratory has 

 produced artificial conditions of temperature enabling 

 him to change the state of the most persistent gases. 

 Some fifty years since, when the kinetic theory was in 

 its infancy, Faraday liquefied carbonic acid gas, among 

 others, and the experiments thus inaugurated have been 

 extended by numerous more recent investigators, notably 

 by Cailletet in Switzerland, by Pictet in France, and by 

 Dr. Thomas Andrews and Professor James Dewar in 

 England. In the course of these experiments not only 

 has air been liquefied, but hydrogen also, the most subtle 

 of gases ; and it has been made more and more apparent 

 that gas and liquid are, as Andrews long ago asserted, 

 " only distant stages of a long series of continuous phys- 

 ical changes." Of course if the temperature be lowered 

 still further, the liquid becomes a solid ; and this change 

 also has been effected in the case of some of the most 

 " permanent " gases, including air. 



The degree of cold that is, of absence of heat thus 

 produced is enormous, relatively to anything of which 

 we have experience in nature here at the earth now, 

 yet the molecules of solidified air, for example, are not 

 absolutely quiescent. In other words, they still have a 

 temperature, though so very low. But it is clearly con- 



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