SOLID HYDROGEN. 257 



contained hydrogen under a pressure of 269.8 millimeters, and the 

 other under a pressure of 127 millimeters. The mean temperature of 

 the solid was found to be 16° absolute under a pressure of 35 milli- 

 meters. All the attempts made to get an accurate electric resistance 

 thermometer for such low temperature observations have been so far 

 unsatisfactory. Now that pure helium is definitely proved to be more 

 volatile than hydrogen, this body, after passing through a spiral glass 

 tube immersed in solid hydrogen to separate all other gases, must be 

 compared with the hydrogen thermometer. Taking the boiling poin v 

 as 21° absolute under 760 millimeters, and the similar value under 31 

 millimeters is 16° absolute, then the following approximate formula 

 for the vapor tension of liquid hydrogen below one atmosphere is 

 derived: 



log^=6.7341-83.28/T mm., 



where T is the absolute temperature, andj? the pressure in millimeters. 

 This formula gives for 55 mm. a temperature of 16.7° absolute. The 

 melting point of hydrogen must therefore be about 16° or 17° abso- 

 lute. It has to be noted that the pressure in the constant volume 

 hydrogen thermometer, used to determine the temperature of solid 

 hydrogen boiling under 35 mm., had been so far reduced that the 

 measurements were made under from one-half to one-fourth the 

 saturation pressure for the temperature. When the same thermom- 

 eters were used to determine the boiling point of hydrogen at atmos- 

 pheric pressure, the internal gas pressure was only reduced to 

 one-thirteenth the saturation pressure for the temperatures. The 

 absolute accuracy of the boiling points under diminished pressure 

 must be examined in some future paper. The practical limit of tem- 

 perature we can command by the evaporation of solid hydrogen is 

 from 14° to 15° absolute. In passing it may be noted that the critical 

 temperature of hydrogen being 30° to 32° absolute the melting point 

 is about half the critical temperature. The melting point of nitrogen 

 is also about half its critical temperature. The foam-like appearance of 

 the solid, when produced in an ordinary vacuum vessel, is due to the 

 small density of the liquid and the fact that rapid ebullition is sub- 

 stantially taking place in the whole mass of liquid. The last doubt as 

 to the possibility of solid hydrogen having a metallic character has 

 been removed and for the future hydrogen must be classed among 

 the nonmetallic elements. 



All solid bodies by themselves make very unsatisfactory cooling 

 agents unless we can use them to cool some liquid. Now, with solid 

 hydrogen we can cool no liquid other than hydrogen, so that, for 

 effective cooling we must use the liquid just above its freezing point, 

 which is about 16°. It will, however, take a long time to exhaust the 

 wide field of investigation which the use of liquid hydrogen opens up, 

 so we may proceed to illustrate some of its further applications. In 

 SM 1901 17 



