8 Professor Dewar [Jan. 20, 



of the vacuum vessel is maintained at about — 190°, the hydrogen 

 is found to evaporate at a rate not far removed from that of liquid 

 air from a similar vacuum vessel under the ordinary conditions of 

 temperature. This leads me to the conclusion that, with proper 

 isolation, it will be possible to manipulate liquid hydrogen as easily as 

 liquid air. 



The boiling point of liquid hydrogen at atmospheric pressure in 

 the first instance was determined by a platinum-resistance thermometer. 

 This was constructed of pure metal and had a resistance of 5 • 3 ohms 

 at 0° C, which fell to about ■ 1 ohm when the thermometer was 

 immersed in liquid hydrogen. The reduction of this resistance to 

 normal air thermometer degrees gave the boiling points — 238 "2° 

 and —238*9° respectively by two extrapolation methods, and —237° 

 by a Dickson formula.* The boiling point of the liquid seems 

 therefore to be — 238° C. or 35° absolute, and is thus about 5° higher 

 than that obtained by Olszewski by the adiabatic expansion of the com- 

 pressed gas, and about 8° higher than that deduced by Wroblewski 

 from Van der Waal's equation. From these results it may be inferred 

 that the critical point of hydrogen is about 50° absolute, and that 

 the critical pressure will probably not exceed 15 atmospheres. 



If we assume the resistance reduced to zero, then the temperature 

 registered by the electric thermometer ought to be — 244° C. At 

 the boiling point of hydrogen, registered by the electric-resistance 

 thermometer, if the law correlating resistance and temperature can be 

 pressed to its limits, a lowering of the boiling point of hydrogen by 5° 

 or 6° C. would therefore produce a condition of affairs in which the 

 platinum would have no resistance, or would become a perfect con- 

 ductor. Now we have every reason to believe that hydrogen, like 

 other liquids, will boil at a lower temperature the lower the pres- 

 sure under which it is volatilised. The question arises, how much 

 lowering of the temperature can we practically anticipate ? For 

 this purpose we have the boiling point given by the hydrogen gas thermo- 

 meter, and critical data available, from which we can calculate an 

 approximate vapour pressure formula, accepting 22° absolute as about 

 the boiling point, 33° absolute as the critical temperature, and 15*4 

 atmospheres as the critical pressure ; then, as a first approximation — 



log. p = 6-410 — — mm. . . . (1) 



If, instead of using the critical pressure in the calculation, we 

 assume the molecular latent heat of hydrogen to be proportional to the 

 absolute boiling point, then from a comparison with an expression of 

 the same kind, which gives accurate results for oxygen tensions below 

 one atmosphere, we can derive another expression for hydrogen vapour 



* See Phil. Mag., 45, 525, 1898. 



