186 THE SUN — ITS CHEMICAL ANALYSIS. 



or liquid, always presents traces of polarisation. The flame of a ^SiS 

 ill combustion alone presents the properties of normal light. This 

 observation furnished a very simple means of discovering the physi- 

 cal constitution of the sun. If it were composed of an incandescent 

 liquid mass it ought to transmit polarized light. If the photosphere 

 were gaseous, the contrary ought to take place. Now, in regarding 

 the sun with a polariscope, Arago found no indication of polarization; 

 thence he concluded that the luminous part of the sun is gaseous, and 

 not liquid or solid. 



The two atmospheric envelopes, which, as Herschel and Arago 

 agree in admitting, exist around an opaque central sun, possess a 

 probable thickness of about 4,000 kilometres; but the singular phe- 

 nomena observed during the total eclipse of the sun of the 8th of July, 

 1842, obliged astronomers to recognize the existence of still a third 

 solar atmosphere above the photosphere properly so called. These 

 phenomena were perceived anew during the eclipse of the year 1860. 

 At the moment when the moon has entirely covered the luminous 

 solar disc, the lunar screen is surrounded by a brilliant luminous 

 aureola, to which has been given the name of corona ; from the border 

 of the moon are projected elevations or protuberances, which ob- 

 servers compare sometimes to serrated rose-colored mountains, some- 

 times to masses of ice tinted red, sometimes to immovable red 

 flames. These protuberances are of a height which may reach to 

 80,000 kilometers, a distance which surpasses even the diameter of 

 the sun. The old theory of Herschel can render no account of these 

 strange appearances. If the sun had the photosphere for its exterior 

 envelope, the sky ought to be completely obscure at the moment 

 when the lunar disc entirely covers the luminous body. We have 

 been compelled, therefore, to admit that there exists a third atmos- 

 phere, of considerable transparency and vast extent, which encompasses 

 the photosphere itself. 



It was impossible that M. KirchofF, in the sequel of the admirable 

 discoveries which he had made with M. Bunsen, should not think of 

 reforming the astronomical doctrines relating to the central star of our 

 planetary system. He has arrived at the conclusion that the visible 

 disc of the sun is not formed by a photosphere; he does not believe 

 in the two envelopes of Herschel and in the opaque central nucleus; 

 he regards the sun as an incandescent body of which the outlines are 

 those of the luminous globe itself, which our eye perceives, and which 

 is surrounded by an immense atmosphere rich in substances of the 

 greatest diversity. The old theory was entirely founded on the ap- 

 pearances of the spots. M. Kirchoft' considers them as clouds floating 

 in the solar atmosphere; he admits that these clouds may be formed 

 at different heights, as occurs in our own atmosphere. Two super- 

 posed clouds, of unequal extent, appear to us far oif as a dark spot 

 encompassed by a penumbra. It cannot be denied that in placing at 

 the centre of the sun an opaque nucleus, and in giving it, for a first 

 atmosphere, a zone half obscure, we present to the understanding an 

 hypothesis which strongl}^ contravenes the instinctive inductions of 

 good sense. If the photosphere is the focus of solar heat and light, 



