230 HISTORY OF COLD AT^D THE ABSOLUTE ZERO. 



is cspcciiilly marked at the negative pole. The spectroscope shows 

 that til is light consists, in the visil)le part of the spectrum, chiefly of 

 a succession of strong ra} s in the red, orange, and yellow, attributed 

 to hydrogen, helium, and neon. Besides these, a vast number of rays, 

 generall}' less brilliant, are distributed through the whole length of 

 the visi))le spectrum. The greater part of these rays are of, as yet, 

 unknown origin. The violet and ultraviolet part of the spectrum 

 rivals in strength that of the red and yellow rays. As these gases 

 prolmldy include some of the gases that pervade interplanetary space, 

 search was made for the prominent neljular, coronal, and auroral lines. 

 No definite lines agreeing with the nebular spectrum could be found, 

 but many lines occurred closely coincident with the coronal and auroral 

 spectrum. But before discussing the spectroscopic problem it will l»e 

 necessary to consider the nature and condition of the upper air. 



According to the old law of Dalton, supported by the modern 

 dynamical theory of gases, each constituent of the atmosphere while 

 acted upon by the force of gravity forms a separate atmosphere, com- 

 pletely independent, except as to temperature, of the others, and the 

 relations between the common temperature and the pressure and alti- 

 tude for each specific atmosphere can be definitely expressed. Jf 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 con- 

 stituents, and in this way the percentage composition of the atmos- 

 phere 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 37 miles, if a sample could be 

 procured for analysis, we believe that it would ]je found to contain 12 

 per cent of hydrogen and only 10 per cent of oxygen. The carl)onic 

 acid practically disappears, and by the time we reach 4T miles, where 

 the temperature is minus 132'. assuming a gradient of 3.2° per mile, 

 the nitrog(>n and oxygen have so thinned out that the only constituent 

 of the upper air which is left is hydrogen. If the gradient of temper- 

 ature were dou])led, the elimination of the nitrogen and oxygen would 

 take place by the time 37 miles was reached, with a temperature of 

 minus 220°. The permanence of the composition of the air at the high- 

 est altitudes, as deduced from the basis of the dynamical theory of gases, 

 has been discussed ))y Stoney, Bryan, and others. It would appear that 

 there is a consensus of opinion that the rate at which gases like hydro- 

 gen and helium could escape from the earth's atmosphere would be exces- 

 sively 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 regard thorn as lu^cessarily permanent constituents of the 

 upper air. The temperature at the elevations we have been discaissing 

 would not be sufHcient to cause any liquefaction of the nitrogen and 

 oxygen, the pressure being so low. If we assume the mean temper- 



