October 17, 1902.] 



SCIENCE. 



623 



gases, each constituent f)i. the atmosphere 

 while acted upon by the force of gravity 

 forms a separate atmosphere, completely 

 independent, except as to temperature, of 

 the others, and the relations between the 

 common temperature and the pressure and 

 altitude for each specific atmosphere can 

 be definitely expressed. If we assume the 

 altitude and temperature known, then the 

 pressure can be ascertained for. the same 

 height in the case of each of the gaseous 

 constituents, and in this way the percentage 

 composition of the atmosphere at that place 

 may be deduced. Suppose we start with a 

 surface atmosphere having the composition 

 of our air, only containing two ten- 

 thousandths of hydrogen, then at thirty- 

 seven miles, if a sample could be procured 

 for analysis, we believe that it would be 

 found to contain 12 per cent, of hydrogen 

 and only 10 per cent, of oxygen. The 

 carbonic acid practically disappears; and 

 by the time we reach forty-seven miles, 

 where the temperature is minus 132 de- 

 grees, assuming a gradient of 3.2 degrees 

 per mile, the nitrogen and oxygen have so 

 thinned out that the only constituent of 

 the upper air which is left is hydrogen. 

 If the gradient of temperature were 

 doubled, the elimination of the nitrogen 

 and oxygen would take place by the time 

 thirty -seven miles was reached, with a tem- 

 perature of minus 220 degrees. The per- 

 manence of the composition of the air at 

 the highest altitudes, as deduced from the 

 basis of the dynamical theory of gases, has 

 been discussed by Stoney, Bryan and oth- 

 ers. It would appear that there is a con- 

 sensus of opinion that the rate at which 

 gases like hydrogen and helium could es- 

 cape from the earth's atmosphere would 

 be excessively slow. Considering that to 

 compensate any such loss the same gases 

 are being supplied by actions taking place 

 in the crust of the earth, we may safely re- 

 gard them as necessarily permanent con- 



stituents of the upper air. The tempera- 

 ture at the elevations we have been 

 discussing would not be sufficient to cause 

 any liquefaction of the nitrogen and oxy- 

 gen, the pressure being so low. If we 

 assume the mean temperature as about the 

 boiling-point of oxygen at atmospheric 

 pressure, then a considerable amoi;nt of 

 the carbonic acid m«st solidify as a mist, 

 if the air from a lower level be cooled to 

 this temperature; and the same result 

 might take place with other gases of rela- 

 tively small volatility which occur in air. 

 This would explain the clouds that have 

 been seen at an elevation of fifty miles, 

 without assuming the possibility of water 

 vapor being carried up so high. The tem- 

 perature 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 tempera- 

 ture must be above the dew-point of air at 

 that place. At higher elevations, on any 

 reasonable assumption of temperature dis- 

 tribution, we inevitably reach a tempera- 

 ture where the air would condense, just as 

 Fourier and Poisson supposed it would, 

 unless the temperature is arrested in some 

 way 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 forty 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 elevation of nine miles 



