8 PLEASANT WA YS IN SCIENCE.. 



action of the vapour, though exerted to reduce the brightness 

 of particular solar rays or tints, would not affect those rays 

 sufficiently for the spectroscopist to recognize any diminution 

 of their lustre. 



There is another consideration, which, so far as I know, 

 has not hitherto received much attention, but should cer- 

 tainly be taken into account in the attempt to interpret the 

 real meaning of the solar spectrum. Some of the metals 

 which are vaporized by the sun's heat below the photosphere 

 may become liquid or even solid at or near the level of the 

 photosphere. Even though the heat at the level of the 

 photosphere may be such that, under ordinary conditions of 

 pressure and so forth, such metals would be vaporous, the 

 enormous pressure which must exist not far below the level 

 of the photosphere may make the heat necessary for com- 

 plete vaporization far greater than the actual heat at that 

 level In that case the vapour will in part condense into 

 liquid globules, or, if the heat is considerably less than is 

 necessary to keep the substance in the form of vapour, then 

 it may in part be solidified, the tiny globules of liquid metal 

 becoming tiny crystals of solid metal. We see both condi- 

 tions fulfilled within the limits of our own air in the case of 

 the vapour of water. Low down the water is present in the 

 air (ordinarily) in the form of pure vapour ; at a higher level 

 the vapour is condensed by cold into liquid drops forming 

 visible clouds (cumulus clouds), and yet higher, where the 

 cold is still greater, the minute water-drops turn into ice- 

 crystals, forming those light fleecy clouds called cirrus clouds 

 by the meteorologist Now true clouds of either sort may 

 exist in the solar atmosphere even above that photospheric 

 level which forms the boundary of the sun we see. It may 

 be said that the spectroscope, applied to examine matter 

 outside the photosphere, has given evidence only of vaporous 

 cloud masses. The ruddy prominences which tower tens of 

 thousands of miles above the surface of tne sun, and the sierra 

 (or as it is sometimes unclassically called, the chromosphere) 

 which covers usually the whole of the photosphere to a 



