ADDRESS. 39 



researches, and a liquid-air plant is becoming a common adjunct to the 

 equipment of the ordinary laboratory. 



The Upper Air and Auroras. 



The present liquid ocean, neglecting everything for the moment but 

 the water, was at a previous period of the earth's history part of the 

 atmosphere, and its condensation has been brought about by the gradual 

 cooling of the earth's surface. This resulting ocean is subjected to the 

 pressure of the remaining uncondensed gases, and as these are slightly 

 soluble they dissolve to some extent in the fluid. The gases in solution 

 can be taken out by distillation or by exhausting the water, and if we 

 compare their volume with the volume of the water as steam, we should 

 find about 1 volume of air in 60,000 volumes of steam. This would 

 then be about the rough proportion of the relatively permanent gas to 

 condensable gas which existed in the case of the vaporised ocean. Now let 

 us assume the surface of the earth gradually cooled to some 200 degrees 

 below the freezing-point ; then, after all the present ocean was frozen, 

 and the climate became three times more intense than any arctic 

 frost, a new ocean of liquid air would appear, covering the entire surface 

 of the frozen globe about 35 feet deep. We may now apply the 

 same reasoning to the liquid air ocean that we formerly did to the 

 water one, and this would lead us to anticipate that it might contain in 

 solution some gases that may be far less condensable than the chief 

 constituents of the fluid. In order to separate them we must imitate 

 the method of taking the gases out of water. Assume a sample of 

 liquid air cooled to the low temperature that can be reached by its own 

 evaporation, connected by a pipe to a condenser cooled in liquid hydrogen ; 

 then any volatile gases present in solution will distil over with the 

 first portions of the air, and can be pumped off', being uncondensable 

 at the temperature of the condenser. In this way, a gas mixture, con- 

 taining, of tlie known gases, free hydrogen, helium, and neon, has been 

 separated from liquid air. It is interesting to note in passing that the 

 relative volatilities of water and oxygen are in the same ratio as those of 

 liquid air and hydrogen, so that the analogy between the ocean of water 

 and that of liquid air has another suggestive parallel. The total uncon- 

 densable gas separated in this way amounts to about one fifty -thousandth 

 of the volume of the air, which is about the same proportion as the air 

 dissolved in water. That free hydrogen exists in air in small amount 

 is conclusively proved, but the actual proportion found by the process is 

 very much smaller than Gautier has estimated by the combustion 

 method. The recent experiments of Lord Rayleigh show that Gautier, 

 who estimated the hydrogen present as one five-thousandth, has in some 

 way produced more hydrogen than he can manage to extract from pure 

 air by a repetition of the same process. The spectroscopic examination 

 of these gases throws new light upon the question of the aurora and tlie 



