Liquid Hydrogen under Reduced Pressure. 



229 



under 25 mm. pressure ought to reduce the boiling point some 10 C. 

 For some time experiments have been in progress with the object of 

 determining the temperature of hydrogen boiling under about 25 mm. 

 pressure, but the difficulties encountered have been so great, and 

 repeated failures so exasperating, that a record of the results so 

 far reached becomes advisable. The troubles arise from the conduction 

 of heat by the leads ; the small latent heat of hydrogen volume for 

 volume as compared with liquid air ; the inefficiency of heat isolation 

 and the strain on the thermometer brought about 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 pres- 

 sure was reduced to 25 mm., or the thermometer was left imperfectly 

 covered. The apparatus employed will be understood from the figure. 



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 teat-tubes of which B is made, 

 thereby making an opening into the interior at C. The resistance 

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



