1886.] Sun-spot Spectra Observations at Kensington. 353 



mum period of sun-spots when we know the solar atmosphere is 

 quietest and coolest, vapours containing the lines of some of our 

 terrestrial elements are present in sun-spots. The vapours, however, 

 which produce the phenomena of sun-spots at the sun-spot maximum 

 are entirely unfamiliar to us. _J 



The disappearance of the lines of iron, nickel, and titanium, and the 

 appearance of unknown lines as the maximum is reached is shown by 

 curves in fig. 1 given on the next page. 



The results, in my opinion, amply justify the working hypothesis 

 as to the construction of the solar atmosphere which I published some 

 years ago. (" Proc. Roy. Soc," vol. 34, p. 291.) In the region of the J 

 spectrum comprised between 4860 and 5160, I find in the J3ase of 

 iron, to take an instance, that sixty lines were distributed unequally 

 among the spots in 1879 and 1880, many iron lines being visible in 

 every spot. In the last observations, about the maximum, only 

 three iron lines in all are seen among the most widened lines. These 

 three lines also have been visible in four spots only out of the last 

 hundred. The same thing happens with titanium and nickel, and 

 with all the substances for which the reductions are finished. 



I am quite content, therefore, to believe that iron, titanium, nickel, 

 and the other substances very nearly as complex as we know them 

 here, descend to the surface of the photosphere, in the downrush that 

 forms a spot at the period of minimum, but that at the maximum, on 

 the contrary, only their finest constituent atoms can reach it. It may";: 

 also be remarked that these particles which survive the dissociating 

 energies of the lower strata are not the same particles among the 

 constituents of the chemical elements named which give the chronio- 

 spheric lines recorded by Tacchini, Ricco, and myself. 



Having thus found the working hypothesis to which I have referred 

 stand the severe test which the sun-spot observations apply to it, I 

 have gone further, and have endeavoured to extend it in two direc- 

 tions. 



First. I found that the view to which the hypothesis directly leads, 

 that the metallic prominences are produced by violent explosions due 

 to sudden expansions among the cooler matters brought down to form 

 the spots, when they reach the higher temperature at and below the 

 photosphere level, includes all the facts I know touching spot and 

 prominence formation. Thus, for instance, the close connexion 

 between metallic prominences and spots ; the entire absence of metallic 

 prominences with rapid motion from any but the spot-zones ; the fact 

 that the faculee always follow the formation of a spot and never pre- 

 cede it ; that the faculous matter lags behind the spot as a rule ; the 

 existence of veiled spots and minor prominences in regions outside' 

 the spot-zones ; the general injection of unknown substances' into the 



2 B 2 



