120 LIGHT SCIENCE FOR LEISURE HOURS. 



magnesium vapour, but complete expulsions. Only 

 we would venture to substitute for the word expul 

 sion the expression outflow or uprising, since it 

 may well be that these vapours rise by a quiet process 

 resembling evaporation, and not by any action so violent 

 that it could properly be regarded as expulsive. 



In whatever way, however, the glowing vapour of 

 magnesium thus streamed into the envelope of the sun, 

 it would seem that the aspect of our luminary was 

 modified by the process not indeed in a very striking 

 manner, or our observers in England would have noticed 

 the change, yet appreciably. 6 More than one person, 

 says Tacchini, has told me that the light of the sun 

 has not at present its ordinary aspect ; and at the 

 Observatory we have judged that we might make the 

 same remark. The change must be attributed to mag 

 nesium. 



It is impossible to consider attentively the remark 

 able occurrence recorded by Tacchini without being 

 struck by the evidence which it affords of solar muta 

 bility. We know that during thousands of years our 

 sun has poured forth his light and heat upon the 

 worlds which circle around him, and that there has 

 been no marked intermittence of the supply. We hear, 

 indeed, of occasions when the sun has been darkened 

 for a while ; and we have abundant reasons for be 

 lieving that he has at times been so spot-covered that 

 there has been a notable diminution of the supply of 

 light and heat for several days together. Yet we 

 have had no reasons for anticipating that our sun 



