1868.] 



Constitution of the Sun and Stars. 



25 



the photosphere should attain an enormous temperature. It is in contact 

 with the luminous clouds, and would on this account alone be brought to 

 as high a temperature as theirs ; but, beside this, rays of every refrangi- 

 bility are emitted from the hotter region beneath the clouds of an intensity 

 corresponding to the far more consuming heat which there prevails. And 

 if out of this terrific heat all the rays be selected which correspond to all 

 the spectral lines of every gas in the solar atmosphere, they will constitute 

 a body of heat, a small part of which is no doubt spent upon the gauze- 

 like luminous clouds, or absorbed by the intermingled atmosphere, but the 

 bulk of which is poured into the atmosphere overhead. On the other 

 hand the only heat which escapes outwards from this upper atmosphere is 

 the quantity, small in comparison, which is emitted by the.se same spectral 

 rays at the reduced temperatures which correspond to the dusky lines 

 visible in the solar spectrum, or to similar lines lying beyond the limits we 

 can see*. All the rest of the heat received by the superincumbent atmo- 

 sphere is returned by it downwards, and is the measure of the fervid tem- 

 perature which its lowest stratum attains. Thus the atmosphere above 

 the luminous clouds will begin by waxing in temperature, and continues to 

 grow hotter through that interval to which the heat emitted from beneath 

 can in any abundance directly penetrate. At the limit of this space there 

 will be a surface of maximum temperature, after which the heat will very 

 gradually fade off by reason of the conduction, convection, and internal 

 radiation which feed the escape outwards from the upper layers of the 

 successive atmospheres. 



36. It is of importance to observe that if the boundary of any one of 

 the gases that constitute the sun's atmosphere fall within the stratum 

 which is hotter than the luminous clouds, or very close above it, that gas 

 can only exist in a state of such utter attenuation within the stratum that 

 we can scarce expect to detect any lines in the spectrum corresponding to 

 it. The stratum in question rests upon the luminous clouds beneath, and its 

 upper limit is to be defined as that situation in which the temperature has 

 again fallen to the same point at which it stands in the shell of clouds. At 

 all intermediate stations the temperature is higher, or, in other words, the 

 motions of the molecules of the gases are more active. At the upper and 

 under boundaries of the stratum they are equal ; but the pressure, and 

 consequently the density, is somewhat less at the upper station, or, in 

 other words, the molecules of the gases constituting the atmosphere are 

 there a little more separated. Now any gas which comes to an end 

 within the stratum must be unable to maintain itself at the upper surface 



* We should remember that much of the sun's heat lies in this direction ; for 

 the wave-lengths of almost all visible vibrations lie between 4 and 8 seventh-metres, and 

 the invisible rays beyond the extreme lavender probably do not include waves much 

 less in length than 2 seventh-metres, while the obscure heat-rays at the other end of the 

 spectrum have been observed to extend, though with decreasing intensity, until the waves 

 are 18 or 20 seventh-metres long, and probably reach much further. 



