170 



THE CHEMISTRY OF THE SUN. 



[CHAP. 



"It is easy to show also that with increased pressure comes such 

 an increased brilliancy as we get in the sun, where, going inwards, 

 we have first the faint corona, then the hydrogen layers, visible 

 only in an eclipse, then those observed every day by the new 

 method, and last of all, the photosphere itself. 



" There is a simple experiment bearing upon this point. If we 

 take a tube containing hydrogen at a very low pressure, and at 

 the bottom of it some mercury, so long as the mercury is cool the 

 spark passes through nearly pure hydrogen, and the tube is lighted 



FIG. 70. Stratification of the solar atmosphere disturbed by the upheaval of a 



prominence. 



up with only a faint glimmer, the equivalent of the auroral 

 discharge. 



" But if instead of having our mercury vapour almost absent 

 from the tube, in consequence of the low temperature of the liquid 

 supply at the bottom, we drive this liquid mercury at the bottom of 

 the tube into a state of vapour, we find that we not only change 

 the colour of the discharge in the lower regions of the tube, but 

 imitate what we get on the sun itself. As the discharge passes 

 through the denser layers, and renders them incandescent as they 

 are at the sun, the brilliancy increases enormously as the source of 

 the vapour is approached, and in each stratum of different density 

 of the mixed gas and vapour in the tube we have the same increase 



