84 LIGHT SCIENCE FOR LEISURE HOURS. 



the surface from which it has risen, because the forma- 

 tion of vapour is a process in which heat is used up. 

 And therefore, by a well-known law, the spectrum of 

 the light from the white-hot surface shining through 

 the atmosphere will be a rainbow-tinted streak, crossed 

 by the dark lines corresponding to the various elements 

 composing that atmosphere. But as the lighter vapours 

 in this mixed atmosphere ascend, they reach a region 

 of less pressure, and a region where they can part more 

 freely with their heat. Thus, precisely as the cumulus 

 clouds form in our own atmosphere, so would a layer 

 of clouds be formed somewhat low down in the solar 

 atmosphere. But from the upper surface of this layer 

 the vapours of the elements composing the clouds would 

 rise, again to condense at a higher level, much as the 

 light cirrus clouds in our own atmosphere form at a 

 great height above the layer of cumulus clouds. 



The great difference between this process and what 

 takes place in our own atmosphere would consist in 

 the fact that whereas the only kind of cloud which can 

 form in our air is a water-cloud, there can be formed in 

 the solar atmosphere clouds of iron, copper, zinc, and 

 other such elements, each element having its own 

 distinct range, so to speak, within the limits of the 

 solar atmosphere. 



Now with such processes as these going on, we can 

 conceive how rushes of heated gas might from time to 

 time thrust aside the cloud-layers ; and how where this 

 happened we should occasionally recognise the bright 

 lines corresponding to the more intensely heated gas, 



