HISTOKY OF COLD AND THE ABSOLUTE ZERO. 281 



atiiro as al)Out the ])oilinj^ point of oxygen at atmospheric pressure, 

 then a coiisidera])le amount of the carbonic acid nuist solidify as a 

 mist, if tlie air from a lower lev^l be cooled to this temperature; and 

 the same result miglit take place with other gases of relatively small 

 volatilit}' which occur in air. This would explain the clouds that have 

 been seen at an elevation of 50 miles, without assuming* the possi])ility 

 of water vapor being carried up so high. The temperature of the 

 upper air must be above that on the vapor pressure curve corre- 

 sponding to the barometric pressure at the locality, otherwise liquid 

 condensation must take place. In other words, the temperature must 

 be above the dew point of air at that place. At higher elevations, on 

 any reasonal)le assumption of temperature distribution, we inevitably 

 reach a temperature where the air would condense, just as Fourier 

 and Poisson supposed it would, unless the temperature is arrested in 

 some wa}" from approaching the zero. Both ultra-violet absorption 

 and the prevalence of electric storms may have something to do with 

 the maintenance of a higher mean temperature. The whole mass of 

 the air above -10 miles is not more than one seven -hundredth part of 

 the total mass of the atmosphere, so that any rain or snow of liquid or 

 solid air, if it did occur, would necessarily be of a very tenuous descrip- 

 tion. In any case, the dense gases tend to accumulate in the lower 

 strata, and the lighter ones to predominate at the higher altitudes, 

 always assuming that a steady state of equilibrium has been reached. 

 It must be observed, however, that a sample of air taken at an eleva- 

 tion of 9 miles has shown no difference in composition from that at the 

 ground, whereas, according to our hypothesis, the oxygen ought to 

 have been diminished to 17 per cent, and the carbonic acid should also 

 have become much less. This can onl}^ be explained b\" assuming that 

 a large intermixture of different layers of the atmosphere is still taking- 

 place at this elevation. This is confirmed by a study of the motions 

 of clouds about G miles high, which reveals an average velocity of the 

 air currents of some To miles an hour; such violent winds must be the 

 means of causing the intermingling of different atmospheric strata. 

 Some clouds, however, during hot and thundery weather, have been 

 seen to reach an elevation of 17 miles, so that we have direct proof 

 that on occasion the lower layers of atmosphere are carried to a great 

 elevation. The existence of an atmosphen^ at more than loo miles 

 above the surface of the earth is revealed to us by the appearance of 

 meteors and fireballs, and when we can take photographs of the spec- 

 trum of such apparitions we shall learn a great deal about the compo- 

 sition of the upper air. In the meantime Pickering's solitar}' spec- 

 trum of a meteor reveals an atmosphere of hydrogen and helium, and 

 so far this is corrol)orative of the doctrine we have been discuss- 

 ing. It has long been recognized that the aurora is the result of elec- 

 tric discharges within the limits of the earth's atmosphere, but it was 



