5-1 Mr. R. Meldola on a Cause for the Appearance 



before referred to*, that "diffused and reflected light of the 

 outer corona could be caused by sucli bodies (oxygen com- 

 pounds) cooled below the self-luminous point." 



6. The following considerations appear to give support to 

 the view that oxygen extends into regions sufficiently reduced 

 in temperature for combustion to take place : — 



The region which is called the chromosphere is distinguish- 

 able as such through what may bo called an optical accident : 

 it is that zone of incandescent hydrogen which is rendered 

 visible by the telespectroscopo ; the true boundary of the hydro- 

 gen atmosphere lies far above the visible chromosphere ; and 

 from this latter zone outwards the temperature falls off rapidly. 

 Now it has been well established by observation, that metals 

 of great molecular mass, such, for example, as those of the iron 

 group, are frequently thrown high up into the chromosphere f. 

 Thus, if gases of great vapour-density are occasionally injected 

 into the chromosphere, gases composed of molecules of com- 

 paratively small mass, such as those of oxygen and nitrogen, 

 would probably extend permanently into regions far above the 

 chromosphere, and which are therefore at a much lower tem- 

 perature than that zone. 



The elements chiefly concerned in producing selective ab- 

 sorption in sun-spots, as shown by the local thickening of their 

 spectral lines, are all elements of high vapour-density com- 

 pared with oxygen— viz. Na, Mg, Ca, Ba, Fe, Ni, Cr, and Ti: 

 from this it appears that the disturbances producing these 

 phenomena must extend low down in the chromosphere. The 

 band-spectra occasionally seen in the nuclei of sun-spots % 

 appear to indicate that in these regions the temperature is 

 sometimes sufficiently reduced to admit of the formation of 



* Loc. cit. p. 3GG. 



t Lockyer, Proc. Roy. Soc. xviii. ; Young-, Jo urn. Frank. Inst. Sept. 

 18G9 and Oct. 1870; also 'Nature/ vol. iii. p. Ill, and vol. vii. p. 17; 

 Respighi, Atti d. Meal, Accad. d. Line. 1872 ; Tacchini, Comptes Rendus, 

 lxxvi. p. 829; H. 0. Vogel, Beobachtungen, 1872; and numerous other 

 observers. 



% Professor Young states (' Nature,' vii. p. 109) that in the spectrum of 

 a sun-spot he observed "between C and D some very peculiar shadings, 

 terminated sharply at the less refrangible limit by a hard dark line, but 

 fading- out gradually in the other direction at a distance of 8 or 4 Kirch- 

 hoff's scale-divisions." This answers in all respects to the spectrum of a 

 compound body : indeed this excellent observer subsequently suggests 

 that these bands "seem to point to such a reduction of temperature over 

 the spot-nucleus as permits the formation of gaseous compounds by ele- 

 ments elsewhere dissociated." In the spectrum of a sun-spot recently 

 observed at the Royal Observatory, Greenwich (Monthly Notices Roy. 

 Astr. Soc. Nov. 9, 1877), a dark-shaded band was seen at about wave- 

 length <;880 ; "sharp towards the blue and shaded off towards the red. 

 Nothing seen on the sun to correspond with it." 



