A HISTORY OF SCIENCE 



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 De- 

 war 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 physical changes." Of course, if the tem- 

 perature 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 conceivable that a stage might be reached at 

 which the molecules became absolutely quiescent, as 

 regards either translational or vibratory motion. Such 

 a heatless condition has been approached, but as yet 

 not quite attained, in laboratory experiments. It is 

 called the absolute zero of temperature, and is esti- 



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