1900.] on Solid Hydrogen. 477 



the hydrogen froth became less opaque, and the solid hydrogen in the 

 tube B was seen to be a transparent ice, but the surface looked frothy. 

 This fact prevented the solid density from being determined, but the 

 maximum fluid density has been approximately ascertained. This was 

 found to be 0*086, the liquid at its boiling-point having the density 

 0-07. The solid hydrogen melts when the pressure of the saturated 

 vapour reaches about 55 mm. In order to determine the temperature of 

 solidification two constant volume hydrogen thermometers were used. 

 One at0° C. contained hydrogen under a pressure of 269*8 mm., and 

 the other under a pressure of 127 mm. The mean temperature of the 

 solid was found to be 16° absolute under a pressure of 35 mm. 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 im- 

 mersed in solid hydrogen to separate all other gases, must be com- 

 pared with the hydrogen thermometer. Taking the boiling-point 

 as 21° absolute under 760 mm., and the similar value under 35 mm. is 

 16° absolute, then the following approximate formula for the vapour 

 tension of liquid hydrogen below one atmosphere is derived : — 



logp = 6-7341 - 83-28 /T mm., 



where T is the absolute temperature, and p the pressure in mm. 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 thermo- 

 meters were used to determine the boiling-point of hydrogen at 

 atmospheric 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 

 temperature 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 

 substantially 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 non-metallic 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 



