A HISTORY OF SCIENCE 



drogen has been cooled by immersion in refrigerating 

 media of very low temperature that this gas becomes 

 amenable to the law of cooling on expansion. In the 

 apparatus used at University College the coil of com- 

 pressed hydrogen is passed successively through (i) a 

 jar containing alcohol and solid carbonic acid at a 

 temperature of 80 Centigrade; (2) a chamber con- 

 taining liquid air at atmospheric pressure, and (3) 

 liquid air boiling in a vacuum bringing the tempera- 

 ture to perhaps 205 Centigrade before entering the 

 Hampson coil, in which expansion and the self-inten- 

 sive refrigeration lead to actual liquefaction. With 

 this apparatus Dr. Travers succeeded in producing 

 an abundant quantity of liquid hydrogen for use in 

 the experiments on the new gases that were first dis- 

 covered in the same laboratory through the experi- 

 ments on liquid air gases about which I shall have 

 something more to say in another chapter. 



PRINCIPLES AND EXPERIMENTS 



At first blush it seems a very marvellous thing, this 

 liquefaction of substances that under all ordinary con- 

 ditions are gaseous. It is certainly a little startling 

 to have a cup of clear, water-like liquid offered one, 

 with the assurance that it is nothing but air ; still more 

 so to have the same air presented in the form of a white 

 " avalanche snow." In a certain sense it is marvellous, 

 because the mechanical difficulties that have been 

 overcome in reducing the air to these unusual condi- 

 tions are great. Yet, in another and broader view, 

 there is nothing more wonderful about liquid air than 



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