296 J. Dewar — Boiling Point of Liquid Hydrogen. 



Another thermometric substance at our disposal as suitable 

 for determining the boiling point of hydrogen, as hydrogen 

 had been in determining that of oxygen, is helium. The early 

 experiments of Olszewski and my own later ones showed that 

 pure helium is less condensible than hydrogen, and that the 

 production of liquid or solid products by cooling Bath helium 

 to the temperatures of boiling and solid hydrogen was only 

 partial, and resulted from the presence of other gases undefined 

 at the time the first experiments were made. The mode of 

 separating the helium from the gases given off by the King's 

 Well at Bath is fully described in my paper on " The Lique- 

 faction of Air and the Detection of Impurities."* 



If the neon, present as impurity in the Bath helium which 

 was used, should reach its saturation pressure about the boiling 

 point of hydrogen the values given by this thermometer of the 

 boiling point of hydrogen would be too low. In order to avoid 

 this, the crude helium extracted from the Bath gas was passed 

 through a U-tube cooled by liquid hydrogen to condense out 

 the known impurities, oxygen, nitrogen and argon. In my 

 paper "On the Application of Liquid Hydrogen to the produc- 

 tion of High Vacua," f it was shown that at the temperature of 

 boiling hydrogen, oxygen, nitrogen and argon have no meas- 

 ureable tension of vapor, and that the only known gases 

 uncondensed in air after such cooling were hydrogen, helium, 

 and neon. This same neon material occurs in the gas derived 

 from the Bath wells. A sample of helium prepared as above 

 described, which had been passed over red-hot oxide of copper 

 to remove any hydrogen, was found by Lord Rayleigh to 

 have a refractivity of 0*132. The refractivity of Ramsay's 

 pure helium being 0*1238, and that of neon 0*2345, it results 

 that my helium contained some 7*4 per cent of neon, accord- 

 ing to the refractivity measurements. This would make the 

 partial tension of the neon in the helium thermometer cooled 

 in the liquid hydrogen to be about 4 mm., and this being taken 

 as the saturation pressure the boiling point of neon is about 

 34° absolute. The initial pressure (No. 9) was taken rather 

 less than an atmosphere, and the temperature of the boiling 

 point of hydrogen was given by this thermometer as — 252°*68. 

 A further observation was taken on another occasion with the 

 same thermometer, and the value was — 252 0, 84. The fact 

 that the boiling point of hydrogen, as determined by the 

 helium thermometer, is in substantial agreement with the 

 results obtained by the use of hydrogen itself is a conclusive 

 proof that no partial condensation of the neon had occurred. 



*Chem. Soc. Proa, 1897. fRoy. Soc. Proa, 1898. 



