252 SOLID HYDROGEN. 



imao-G of the cup containing liquid hydrogen, covered loosely in this 

 case with a glass plate, upon the screen; here no heavy vapor escaping 

 round the sides is visible. The vapor of the boiling liquid hydrogen 

 has a densitv nearly equal to the air of the room, but as it gets very 

 rapidly heated up by the glass cover the gas that is escaping is seen to 

 rise in air like any light gas. On now removing the glass plate a very 

 different phenomenon is observed, which contrasts markedly with the 

 Ix'havior of the liquid air in the former vessel. The cup and the air 

 a])ove is filled with a dense surging snowstorm of solid air; the 

 air, coming in contact with the excessively cold hydrogen vapor, is 

 suddenly solidified, and a part of it falls into the liquid hydrogen, 

 causing more rapid evaporation, thereby intensifying the cloud con- 

 densation. After the mist has disappeared and all the liquid hydrogen 

 gone the cup contains a white deposit of solid air. This shortly melts, 

 and on allowing the nitrogen to boil off, the presence of oxygen can be 

 shown by the ignition of a red-hot splinter of wood. Such effects are 

 easily understood when we remember that the boiling point of hydro- 

 gen is proportionall}^ as much below the boiling point of air as the 

 latter is below the ordinary temperature of this room. 



In order to o])serve the individual behavior of the constituents of 

 the air at temperatures below their ordinary boiling points, it is 

 advantageous to place liquid nitrogen and ox3'gen in separate vacuum 

 vessels, so connected that they may l)o simultaneously exhausted, as is 

 represented in fig. 4. On starting the air pump both liquids enter 

 into rapid ebullition. As the exhaustion gets higher the temperature 

 of each liquid gets lower and lower, and if the melting point is finally 

 reached in either liquid it must shortl}^ begin to solidify. This condi- 

 tion is quickly brought about in the case of the vessel A, containing 

 the liquid nitrogen, which passes rapidly into the condition of a dense 

 white snow; but no amount of time spent in maintaining a good 

 exhaustion (.5 to 10 millimeters pressure) has any effect in changing the 

 liquid condition of the oxygen in B. Oxygen in fact remains liquid at 

 temperatures where nitrogen is solid. The snow of solid air produced 

 by the evaporation of liquid hydrogen in the previous experiment 

 might thus be made up of solid nitrogen and a liquid rain of oxygen. 

 To show that the temperature of boiling hydrogen solidifies oxygen, 

 some of the latter liquid is placed in a vacuum test tube O (fig. 3) and 

 liquid hydrogen H is poured on its surface, when the liquid oxygen 

 is quickly transformed into a clear blue solid ice. Both oxygen and 

 nitrogen, and we shall see later hydrogen, can be changed into the con- 

 dition of transparent ice as well as into the snowy state. A closed 

 vessel filled with anj^ gas at atmospheric pressure, of such a form that 

 a portion of the surface in the shape of a narrow quill tube, can be 

 cooled in boiling liquid hydrogen like B, fig. 5, shows condensation of 

 the gas to the solid state, the only exceptions being helium and hydro- 



