On the Formation of Solids at Low Temperatures. 181 



Note on the Formation of Solids at Low Temperatures, particu- 

 larly with Eegard to Solid Hydrogen. " By Morris W. 

 Travers, D.Sc, Professor of Chemistry at University College, 

 Bristol. Communicated by Sir W. Ramsay, K.C.B., F.B.S. 

 Beceived February 4, — Bead February 18th, 1904. 



In the year 1902 Dr. Jaquerod and I carried out some experiments 

 mi liquid and solid hydrogen with a view to determining its vapour 

 pressure on the scales of the constant-volume helium and hydrogen 

 thermometers. We found that hydrogen remained liquid down to 

 14° '2 (He scale), the lowest temperature to which we could reduce a 

 large mass of the liquid by means of the pump at our disposal. When, 

 however, a small quantity of liquid hydrogen, cooled to 14° -2 in a 

 glass tube immersed in the liquid contained in the large vacuum vessel, 

 was allowed to evaporate under reduced pressure, it solidified when 

 the pressure fell to 19 or 50 mm. of mercury. This pressure 

 corresponds to a temperature of 14° -1 on the helium scale. The 

 presence of the solid was determined by mechanical means, and it was 

 not possible to observe its appearance.* 



Dewar gives the melting point of hydrogen at about 15° absolute, 

 •and the melting pressure at 55 mm. of mercmy. He describes its 

 appearance as that of " frozen foam," or as " clear transparent ice."f 



It appeared to me worth while to carry out a few experiments to 

 try to determine whether solid hydrogen formed definite crystal, or 

 indeed whether the glassy substance was a true solid or merely a 

 highly viscous fluid. My meaning will become clearer if I give an 

 instance in which both such changes occur. 



If an organic liquid, such as ethyl aceto-acetate, is cooled slowly to the 

 temperature of liquid air, it is converted into crystalline solid, the 

 •formation of the crystals commencing when the liquid is cooled to 

 about - 150° C, usually at several points on the side of the vessel, and 

 spreading rapidly throughout the mass. If, on the other hand, the 

 liquid is cooled very rapidly, a hard glassy substance is formed, and 

 though crystals may begin to appear, they will only do so locally, as 

 the velocity of crystallisation decreases rapidly as the viscosity of the 

 liquid increases. The glassy substance is really a liquid of high 

 viscosity ; it is formed with perfect continuity from the normal liquid 

 state, and should differ from the solid (crystalline) form in its physical 

 properties. Such a substance might, for convenience, be called a 

 pseudo-solid. 



* ' Phil. Trans.,' A, vol. 200, p. 170. 



t British. Association, Presidential Address, 1902. See also paper on "Solid 

 Hydrogen," ' Brit. Assoc. Report.' 1899, reprinted in 'Nature'; also ' Roy. Inst. 

 Proc.,' 1900. 



VOL. LXXIII. O 



