1908] on the Nadir of Temperature and Allied Problems. 421 



incandescence again establishes itself. The pressure still increasing, 

 the incandescence again dies down and disappears. On opening the 

 cock to the charcoal the same phenomena are again repeated. 



Fig. 5. 



The limits of pressure, as measured l)V a McLeod gauge, within 

 which this incandescence (u'curs ran<>-e from O'OS mm. to O'OO:) mm. 



Solid HYDRO(iEX by Charcoal Exhaust. 



A liquid may be solidified by the aid of a charcoal condenser 

 maintained at four times its absolute boihng point. A vacuum tube 

 (Fig. 6) containing liquid hydrogen A is surrounded by another 

 vessel of liquid air B, arranged for projection, care being taken to 

 guard against heat radiation from the electric lamp by the use of 

 a water cell. A large bulb of charcoal C, cooled in a sepai'ate vessel 

 of liquid air F, is connected to a bent tube provided with a three- 

 way stopcock D, passing through an india-rubber cork E which fits 

 the liquid hydrogen vacuum tube A. Tlie cork is now rajMdly fitted 



