at the Royal Institution, 1908-1916 



767 



In this connexion, it is desirable to note the superior vaUie of the 

 gas thermometer as a means of measuring very low temperatures. 

 The practical limits of temperature reached by the volatilization at 



Sea/ efter cono/ens/ng ox(/ge/i 



M^Oeod gauge 



to 

 Mercury 



Pump 



Fig. 3. 



A. Silvered vacuum-jacketed tube, with an uncoated slit, containing 

 5-10 cc. oxygen. B. Silvered vacuum tube, with clear slit, con- 

 taining liquid air under an exhaust less than 20 mm. D. Charcoal 

 in boiling oxygen. 



low pressures of various substances are shown in the following table ; 

 it is to be noted that the maximum cooling effect is. in all cases, one- 

 third of the critical temperature of the substance volatihzed : — 



Limits of Temperature obtainable with various Cooling Agents. 



