LIQUID HYDROGEN. 137 



pose w.e have the boiling point given by the hydrogen-gas thermometer 

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

 vapor-pressure formula, accepting 22° absolute as al)0ut the boiling 

 point, 33° absolute as the critical temperature, and 15.4 atmospheres 

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



77.62 

 log. p — 0. iK ) — — rp~ 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 abso- 

 lute 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 vapor 

 pressures which ought to be applicable to boiling points under reduced 

 pressure. 



The resulting formula is — 



88 

 log. ^=7.0808— 7p mm (2) 



Now formula (1) gives a boiling point of 14.2° absolute under a pres- 

 sure of 25 millimeters, whereas the second equation (2) gives for the 

 same pressure 15.4° absolute. As the absolute boiling point under 

 atmospheric pressure is about 22°, both expressions lead to the concki- 

 sion that ebullition under 25 millimeters pressure ought to reduce the 

 boiling point some 7° C For some time experiments have been in 

 progress with the object of determining the temperature of hydrogen 

 boiling under about 25 millimeters pressure by the use of the platinum 

 thermometer; but the difficulties encountered have been great, and 

 repeated failures very exasperating. The troubles arise from the con- 

 duction of heat by the leads, the small latent heat of hydrogen volume 

 for volume as compared with liquid air, the inefficiency of heat isola- 

 tion, and the strain on the thermometer l)rought al)Out by solid air 

 freezing on it and distorting the coil of wire. In many experiments 

 the result has been that all the liquid hydrogen has evaporated before 

 the pressure was reduced to 25 millimeters, or the thermometer w^as 

 left imperfectly covered. The apparatus employed will be iniderstood 

 from fig. 2. The liquid hydrogen collected in the vacuum vessel A was 

 suspended in a larger vessel of the same kind B, which is so constructed 

 that a spiral tube joins the inner and outer test tubes of which B is 

 made, thereby making an opening into the interior at C. The resist- 

 ance thermometer D and leads E pass through a rubber cork F, and 

 the exhaustion takes place through C. In this way the cold vapors are 

 drawn over the outside of the h^^drogen vacuum vessel and this helps 

 to isolate the liquid from the convective currents of gas. To effect 

 proper isolation, the whole apparatus ought to be immersed in liquid 



