256 SOLID HYDROGEN. 



hydfoocn. This .surnii.se was confinnod by ol)servincr that if the pres- 

 sure, and therefore the temperature, of the hydrogen was allowed to 

 rise, the solid melted when the pressure reached about 55 millimeters. 

 The failure of the early experiment nuist then have been due to super- 

 cooling- of the liquid, which presumably is prevented by contact with 

 metallic wires and traces of solid air. On the other hand, it is possi- 

 ble the pressure under which the ebullition took place might never 

 have been low enough to reach the solid state. 



For the lecture demonstration of solid hydrogen the apparatus may 

 be most conveniently arranged as is shown in tig. 7. The small vacuum 

 tube B, after being tilled with liquid hydrogen, is immersed in a larger 

 vessel of the same kind filled with liquid air. By this arrangement the 

 rate of the liquid hydrogen evaporation is so much diminished that it 

 does not exceed that of liquid air in the same vessel when used in the 

 ordinary way. On gradually appljdng exhaustion to the liquid hydro- 

 gen it is forced from its effective heat isolation to pass to a lower tem- 

 perature, and when the exhaustion reaches 50 millimeters the mass sud- 

 denly begins to solidify into a froth-like material. In order to ascertain 

 the appearance of the hydrogen, made ))y cooling the liquid produced 

 in a hermetically closed vessel, the following experiment was arranged. 

 A ffask about a liter capacity, to which a long glass tube was sealed, 

 A, B, fig. 5, was tilled with pure dry hydrogen and sealed off'. The 

 lower portion B of this tube was calibrated. It was surrounded with 

 liquid hydrogen placed in a vacuum vessel arranged for exhaustion. 

 As soon as the pressure of the boiling hydrogen got well reduced 

 below that of the atmosphere, perfectly clear liquid hj'drogen began 

 to collect in the tube B, and could be observed accumulating until the 

 liquid h3'drogen surrounding the outside of the tube suddenly passed 

 into a solid white foam-like mass, almost filling the whole space. As 

 it was not possible to see the condition of the hydrogen in the interior 

 of the tube B when it was covered with a large quantit}'^ of this solid, 

 the whole apparatus was turned upside down in order to see whether 

 any liquid would run down from B into the flask A. Liquid did not 

 flow down the tu])e, so the liquid h3'drogen with which the tube was 

 partly filled must have solidified. By placing a strong light on the side 

 of the vacuum test tube opposite the eye, and maintaining the exhaus- 

 tion at about 25 millimeters gradually the hA'drogeu froth became less 

 opaque, and the solid hj^drogen in the tube B was seen to be a trans- 

 parent ice, but the surface looked frothy. This fact prevented the 

 solid density from being determined, Init the maxinunn 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 hydro- 

 gen melts when the pressure of the saturated vapor reaches about 55 

 millimeters. In order to determine the temperature of solidification 

 two constant volume hydrogen thermometers were used. One at 0^ 0. 



