SOLID HYDROGEN. 259 



pure copper wire when cooled in liquid hydrogen in contrast t^ liquid 

 air. An incandescent lamp C has been placed in circuit with a tine 

 coil of copper wire A, immersed in liquid air, the resistances being so 

 adjusted that the filament in C is just visible when the current passes 

 under these conditions. Now, on removing the coil from the liquid- 

 air vessel and placing it in another similar vessel tilled with liquid 

 hydrogen, a great increase in the brilliancy of the lamp is observed. 

 As a matter of fact, the sample of copper has its resistance in liquid 

 air reduced to about one-twentieth of what it is at the temperature of 

 melting ice, whereas in liquid hydrogen the resistance is reduced to 

 one-hundredth of the same amount. In other words, the resistance in 

 liquid hydrogen is only about one-tifth of what it is in liquid air. The 

 interesting point, however, is that theoretically we should infer, 

 from experiments made at higher temperatures, that at a temperature 

 of —223° C. the copper should have no resistance or it should have 

 become a perfect conductor. As this is not the case, even at the 

 temperature of —253°, we must infer that the curve corelating resist- 

 ance and temperature tends to become asymptotic at the lowest 

 temperatures. 



Liquid hydrogen is a most useful agent for the production of high 

 vacua and for the separation of gases from air that may be more vol- 

 atile than oxygen or nitrogen. An experiment illustrating the produc- 

 tion of a high vacuum is shown in fig. 10, where A is the large electric 

 discharging tube, to which has been attached a narrow glass tube twice 

 bent at right angles and terminating in a bulb at the end for immer- 

 sion in the liquid hydrogen. The rapidity with which the vacuum is 

 attained is shown by the rate at which the striation in the tube 

 changes and the phosphorescent state supervenes. Another rough 

 illustration of the application of cold to effect the separation of a com- 

 plex mixture of gases is shown in fig. 8. Coal gas is passed in suc- 

 cession through the U -tubes F, G, and H, made of ordinary gas-pipe, 

 having small holes at B, C, D, and E, in order that a flame may be 

 produced before and after each vessel is passed. Each of the U -tubes 

 is placed in a vacuum vessel, and the first cooling substance the gas in 

 its transit meets is solid carbonic acid in F, then liquid air in G, and 

 finally liquid hydrogen in H. At the temperature of the carbonic- 

 acid bath all the easily condensable hydrocarbons separate, and conse 

 qently the flame C is less luminous than B. The liquid-air bath con- 

 denses the eth} T lene and a large part of the marsh gas and allows the 

 carbonic oxide and the hydrogen to pass through, so that flame D is less 

 luminous than C. Finally, after the liquid-hydrogen bath, nothing 

 escapes condensation but free hydrogen, the carbonic oxide and any 

 marsh gas being solidified; the result is, the flame E is almost invisible. 



A really practical application of liquid hydrogen is the purification 

 of helium obtained from the gases emitted by the mineral springs of 

 Bath. Although the helium only amounts to one-thousandth part by 



