478 Professor Dcwar [April 6, 



effective cooling, we must use the liquid just above its freezing-point, 

 which is about 16°. It will, however, take a long time to exhaust 

 the wide field of investigation which the use of liquid hydrogen 

 opens up; so we may proceed to illustrate some of its further 

 applications. In former lectures the relation of electr'cal resistance 

 to temperature has been discussed, and it was experimentally 

 demonstrated that the curves of resistance of the pure metals all 

 pointed to this quality disappearing or becoming exceeding small 

 at the absolute zero. This fact has been confirmed, even with tho 

 most highly conducting metals, down to the lowest temperature we 

 can command. The experiment illustrated in Fig. 9 shows to an 

 audience the diminution of resistance of pure copper wire when 

 cooled in liquid hydrogen, in contrast to liquid air. An incan- 

 descent lamp C has been placed in circuit with a fine 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 filled 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-fifth 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 — L ; 53°, we must infer that the curve co-relating 

 resistance 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 

 volatile than oxygen or nitrogen. An experiment illustrating the 

 production 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 immersion in the liquid hydrogen. The rapidity with which the 

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

 the tube ehanges and the phosphorescent state supervenes. Another 

 rough illustration of the application of cold to effect the separation of 

 a complex mixture of gases is shown in Fig. 8. Coal-gas is passed in 

 succession 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 sub- 

 stance tho gas, in its transit meets is solid carbonic acid in F, then 

 liquid air in G, and finally liquid hydrogen in H. At the tempera- 

 ture of the carbonic acid bath, all the easily condensable hydro- 



