32 PRESIDENT’S ADDRESS. 
The lines of hydrogen, helium, and asterium are found iv 
the nebule with certain other bright lines’ of unknown 
origin; these probably exist at a low temperature. The 
unknown stellar lines are being sought for, and may be 
found to occur in gases of terrestrial origin, 
LIQUEFACTION OF GASES. 
Since Faraday’s time no very great advance was made in 
the liquefaction of gases until 1878, when Cailletet, of Paris, 
and Pictet, of Geneva, almost simultaneously, but indepen- 
dently, were successful in liquefying oxygen, nitrogen, and 
other so-called permanent gases. 
Cailletet submitted the cooled gas to very great pressure 
and then suddenly released it, when the remainder of the 
gas became sufficiently cooled by its rapid expansion to 
hquety, and as the operation was conducted in glass tubes 
the formation of the mist and liquid was readily seen. 
Pictet used a different method, viz., that known as the 
cascade or successive cycle process. Tor example, carbon 
dioxide can be liquefied, and this on evaporation yields a 
temperature of —80° C., or less, if its evaporation be increased 
by the use of an air-pump. The liquid carbon dioxide can 
thus be used to cool another and more volatile liquid, say 
ethylene, which on evaporation yields a reduction in tem- 
perature of —130°. Next the liquid ethylene is used for 
cooling a gas still more difficult to liquefy, say oxygen. 
The oxygen having been cooled down to —130° C. is readily 
condensed to the liquid condition by a pressure of 20 or 30 
atmospheres. On releasing the pressure and allowing the 
oxygen to evaporate a reduction of —200° C. (when evapo- 
rating quietly at ordinary pressure it is -185°) is obtained. 
If the evaporation of the liquid oxygen be hastened by the 
use of an air-pump a temperature of —220°, or even —225°, 
may be produced. 
Professor Dewar and others have since used processes in 
which Pictet’s and Cailletet’s methods are more or less 
combined. 
