302 GREAT MEN OF SCIENCE 



be used in a converse sense for still further investigation of 

 the peculiarities of the gas molecule. The specific heats of 

 gases, with their peculiar laws, could also now be under- 

 stood, and thus became a new means for the accurate 

 estimation of the number of atoms in a molecule. It was 

 found possible to distinguish monatomic molecules, such as 

 those of the metallic vapours (and later the rare gases), 

 from di- and poly-atomic molecules. All this was put upon 

 a firm basis by Clausius. The further rapid development of 

 these matters, which was completed as regards essentials 

 within ten years, was due, besides to W. Thomson, to 

 Maxwell, Boltzmann, and Loschmidt.^ 



After this development in our knowledge, an old problem 

 was also finally solved, namely that of the liquefaction of 

 gases. The present method of manufacturing liquid air on 

 a large scale depends upon the discovery made by Thomson 

 in collaboration with Joule in an experimental investigation 

 carried out with great persistence, and v^e will now discuss 

 this. Originally, all the gaseous substances discovered by 

 Scheele, Priestley, and Cavendish gave rise to the funda- 

 mental question whether they could by any means be re- 

 duced to the liquid condition. 



Faraday was the first to work on this problem, which he 

 did in 1823, when still Davy's assistant. He made use of the 

 means which had been known since Dalton's investigations 

 of vapours, for the liquefaction of the latter: pressure and 

 cold. He succeeded in liquefying by simple means chlorine, 

 carbonic acid and other gases. He caused the gases to be 

 generated in sealed glass tubes, and thus produce their own 

 pressure, and cooled one end of the tube, which was bent 

 downwards, in a freezing mixture. It was then found out 

 that this liquefaction could be carried out on a large scale by 



^ Loschmidt was born in 1821, and was the son of poor peasants near 

 Carlsbad in Bohemia; he died in 1895 as professor of physics at the 

 University of Vienna. 



