736 Professor Dewar [Jan. 18, 



or better at a lower temperature by exhausting the surrounding air. 

 Many combinations of vacuum vessels can be arranged, and tbe lower 

 the temperature at which we have to operate the more useful they 

 become. 



As the great object in producing liquid gases is in the first place 

 scientific utility in opening up new fields of research, the application 

 of liquid hydrogen as an agent by means of which the more volatile 

 gases contained in atmospheric air may be separated is of some 

 interest. 



The diagram, Fig. 1, will make the process of separation intel- 

 ligible. A represents a vacuum-jacketed vessel, partly filled with 

 liquid air, in which a second vessel B, was immersed. From the 

 bottom of B a tube, a, passed up through the rubber cork which 

 closed A, and from the top of B a second tube, b, passed through 

 the cork and on to the rest of the apparatus. Each of these tubes 

 had a stopcock, m and n, and the end of the tube a was open to the 

 air. A wider tube also passed through the cork of A and led to an 

 air-pump, whereby the pressure above the liquid air in A was reduced, 

 and consequently the temperature of the liquid by resultant evapo- 

 ration. To keep the inner vessel, B, covered with liquid, a fourth 

 tube, r, passed through the cork, and its lower end, furnished with 

 a valve, p, which could be opened and closed by the handle q, 

 dipped into liquid air contained in the vessel C. As the pressure 

 above the liquid in A was less than that of the atmosphere, on open- 

 ing the valve p some of the liquid air was forced through r into A 

 by the pressure of the atmosphere, and in this way the level of liquid 

 in A was maintained at the required height. 



Since B was maintained at the temperature of liquid air boiling 

 at reduced pressure, the air it contained condensed on its sides, and 

 when the stopcock n was closed and m opened more air passed in 

 through the open end of a, and was in turn condensed. In this way 

 B could be filled completely with liquid air, the whole of the most 

 volatile gases being retained in solution in the liquid. 



The tube b, passing from the top of B, was connected with a three- 

 way stopcock d, by which it could be put in communication with the 

 closed vessel D, or with the tube e, and by which also D and e 

 could be connected. The tube e passed down nearly to the bottom 

 of the vacuum-jacketed vessel E, and out again through the cork ; 

 and so on to a gauge /, and through a sparking tube g to a mercury 

 pump F* The stopcock n being still closed, the whole of the 

 apparatus between n and the pump, including the vessel D, was 

 exhausted, and liquid hydrogen introduced into E. The three-way 

 cock d was then turned so as to connect b with D, and close e, and 

 then n opened. B was thereby put in communication with D, which 

 was at a still lower temperature than B, the air being at 63° absolute, 

 while the hydrogen is at 21° absolute, and any gas dissolved in the liquid 

 air in B, along with some of the more volatile nitrogen, distilled 



* The Sprengel in figure is simply diagrammatic. 



