THE SUN — ITS CHEMICAL ANALYSIS. 185 



a black portion, nearly central, surrounded by a penumbra not so 

 deeply shaded and sliarply defined as regards the darkest zone. The 

 penumbra is generally a little ligliter around the central black spot. 

 It is quite rare to witness a spot without penumbra or a penumbra 

 without a spot. 



When we observe a spot during several days, very curious changes 

 are remarked in the outlines of the black portion and the penumbra. 

 If the spots were adherent to the body of the sun itself and, as La 

 Hire and Maupertuis still believed at the close, of the last century, 

 were species of isles or scums in the solar ocean, we should naturally 

 see first one part of the penumbra vanish, then the black cent^ral 

 nucleus, then the other side of the penumbra. It is observed, on the 

 contrary, that as the spot advances towards the western border of 

 the solar disc, the western penumbra, instead of diminishing, in- 

 creases in size, the eastern penumbra shrinks and is effaced ; lastly, 

 the black central nucleus itself disappears before the western penum- 

 bra. In reference to this observation, which is due to Wilson, and 

 to give an explanation of it, William Herschel, in 1779, surmised 

 that the sun has an envelope of a nature altogether peculiar. In 

 the centre he supposed there might be a solid and opaque nucleus, 

 surrounded on all sides by a gaseous and transparent atmosphere, as 

 in the case of the terrestrial atmosphere; this might be composed of 

 two strata : an exterior luminous one, the true photosphere of the 

 sun; and a'lower stratum, obscure or feebly illuminated by reflection. 

 How, by this hypothesis, are we to explain the phases of the solar 

 spots? Imagine that a hurricane, rends the atmosphere over an 

 immense expanse, (there are spots whose extent is not less than the 

 whole surface of the earth ;) at the bottom of the gulf thus formed, 

 the terrestrial observer will perceive the solid nucleus of the sun as 

 a dark spot, and the first atmosphere, slightly transparent and faintly 

 illuminated, as a penumbra encircling the central spot. It will thus 

 be easily comprehended that the movement of rotation of the sun 

 will, by presenting the object obliquely, withdraw from our view one 

 of the sides and the bottom of this vast chasm before withdrawing 

 the opposite side. All the phenomena described by Wilson are thus 

 very easily accounted for. But by the influence of what forces is it 

 that the luminous veil of the sun and the second semi-diaphanous veil 

 are torn apart to show us the opaque nucleus ? This no one has been 

 able to explain. Herschel supposed that the solid nucleus was cov- 

 ered with volcanoes, whose vapors, discharged with great force, might 

 break up through the solar atmosphere; but this is a supposition 

 wholly gratuitous, which nothing has seemed to confirm.'* 



A discovery made by Arago, in 1811, seemed to give a new degree 

 of probability to the strange hypothesis of Herschel on ihe consti- 

 tution of the sun. It had been long believed that the light emitted 

 by incandescent bodies was not polarized ; Arago observed, on the 

 contrary, that the light proceeding from an incandescent body, solid 



* Phenomena lately observed indicate that the nucleus of the sun, as well as its atmo- 

 sphere, is subjected to violent commotions. J. H. 



