476 "Professor Dcwar [April 6, 



through indiaruhber corks it is very difficult at these excessively 

 low temperatures to prevent leaks, when corks get as hard as 

 a stone, and cements crack in all directions. The effect of this 

 slight air leak on the liquid hydrogen when the pressure got reduced 

 below 60 mm., was very remarkable, as it suddenly solidified into a 

 white froth-like mass like frozen foam. My first notion was that 

 this body might be a sponge of solid air containing liquid hydrogen. 

 The ordinary solid air obtained by evaporation in vacuo is a magma 

 of solid nitrogen containing liquid oxygen. The fact, however, that 

 this white solid froth evaporated completely at the low pressure with- 

 out leaving any substantial amount of solid air led to the conclusion 

 that the body after all must be solid hydrogen. This surmise was 

 confirmed by observing that if the pressure, and therefore the tem- 

 perature, of the hydrogen was allowed to rise, the solid melted when 

 the pressure reached about 55 mm. The failure of the early ex- 

 periment must then have been due to supercooling of the liquid, 

 which presumably is prevented by contact with metallic wires and 

 traces of solid air. On the other hand, it is possible 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 Fig. 7. The small 

 vacuum tube B, after being filled 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 applying ex- 

 haustion to the liquid hydrogen it is forced from its effective heat 

 isolation tr> pass to a lower temperature, and when the exhaustion 

 reaches 50 mm. the mass suddenly begins to solidify into a froth-like 

 material. In order to ascertain the appearance of the hydrogen, made 

 by cooling the liquid produced in a hermetically closed vessel the 

 following experiment was arranged. A flask of about a litre capacity, 

 to which a long glass tube was sealed, A B, Fig. 5, was filled 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, per- 

 fectly clear liquid hydrogen began to collect in the tube B, and could 

 be observed accumulating until the liquid hydrogen surrounding tho 

 outside of the tube suddenly passed into a solid white foam-like mass, 

 almost tilling the whole space. As it was not possible to see the condi- 

 tion of the hydrogen in the interior of the tube B when it was covered 

 with a large quantity 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 tube, so the liquid 

 hydrogen with which the tube was partly filled must have solidified. 

 By placing a strong light on the side of the vacuum test-tube ojqiosite 

 the eye, and maintaining the exhaustion at about 25 mm., gradually 



