The Liquefaction of Gases. 25 



coils of tube immersed in water, and finally allowed to escape 

 through a specially constructed valve, whereby in expanding it 

 cools itself further, though not enough for the purposes of lique- 

 faction. For this there is necessary the so-called self-intensive 

 principle. Just before the air escapes by the valve it has to 

 pass through a coil containing very many turns of tube 

 contained in a non-conducting casing. Through this casing 

 and among the coils of tube the recently escaped and therefore 

 cold air circulates, imparting its cold to the coils, and therefore 

 to the compressed air circulating in them. We have thus a 

 continuous interacting process of cooling, by which in a short 

 time the air in the coils is reduced to a liquefying temperature. 

 This is the self-intensive principle used by Linde, Dewar, 

 Hampson, etc., for liquefaction of air, and finally by Travers for 

 liquefaction of hydrogen, a much more difficult matter. 



It would be quite impracticable to keep liquid air in closed 

 vessels because of the enormous pressure that would result when 

 the liquid, by absorbing heat from the surrounding atmosphere 

 through the walls of the vessel, would evaporate and regain its 

 gaseous form. In an ordinary open glass vessel it evaporates in 

 time, of course ; but if we reduce the influx of heat by surround- 

 ing the vessel with a non-conducting envelope we may retard this 

 evaporation. The best nonconducting envelope known is a 

 vacuous or highly-exhausted space, consequently what are 

 called vacuum jacketed vessels are employed — i.e., the glass 

 tube or flask containing the liquid is surrounded by a second 

 larger glass tube or flask, sealed on so that the interspace can 

 be exhausted of air. Through this space conduction of heat is 

 very slight. Radiation of heat into the vessel is prevented by 

 silvering the interior between the two vessels, and so reflecting 

 the heat rays that strike upon it. The invention of these 

 vessels is claimed by Professor Dewar. 



For the opportunity of seeing and experimenting with 

 liquefied air we are indebted to the very great kindness of Mr. 

 Richard J. Moss, of the Royal Dublin Society, who with the 

 greatest cordiality acceded to a request from me for a supply of 



