HISTOKY OF COLD AND THE ABSOLUTE ZERO. 229 



THE I ITEK AIR AND AURORAS. 



The present liquid oreaii, neglet'ting eveiything for the inoiuent but 

 the wat-er, was at a previous period of the earth's history part of the 

 atmosphere, and its condensation lias ])een brought about b}' the grad- 

 ual cooling of the earth's surface. This resulting ocean is subji^cted 

 to the })ressure of the remaining uncondensed gases, and as these are 

 slightly soluble they dissohe 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 Avith the volume of the water as steam, 

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

 would then he about the rough proportion of the relatively permanent 

 gas to condensable gas which existed in the case of the vaporized ocean. 

 Now h't us assume the surface of the earth gradually cooled to some 

 200^ below the freezing point; then, after all the present ocean was 

 frozen and the climate l)ecame thrcn^ times more intense than any arctic 

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

 surface of the frozen glol)e about 35 feet deep. We may now appl}' 

 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 

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

 liquid air cooled to the low temperature that can ])e reached ])y its own 

 evaporation, connected by a pipe to a condenser cooled in liquid hydro- 

 gen; then any volatile gases present in solution will distill over with 

 the flrst portions of th(^ air, and can be pumped oft', being uncondensable 

 at the temperatui'e of the condenser. In this way a gas mixture, con- 

 taining, of the 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 

 licpnd air and hydrogen, so that the analogy between the ocean of 

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

 uncondensable gas separated in this way amounts to al)()ut one flfty- 

 thousandth of the volume of the air, which is about the same propor- 

 tion as the air dissolved in water. Tiiat free hydrogen exists in air in 

 small amount is conclusively proxed. I)ut tlu^ actual proportion found 

 by the process is xcry much sinall(>r than (fautim' has estimated by 

 the cond)ustion method. The recent experiments of Lord Rayleigh 

 show that (lautier, who estimated the hydrogen present as on(^ ftve- 

 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 the nature of the upper air. On pass- 

 ing electric discharges through the tul)es containing th(» most volatile 

 of the atmospheric gases, they glow with a bright orange light, which 



