172 ANNUAL OF SCIENTIFIC DISCOVERT. 



bodies, existed in former times as an extended and continuous mass 

 of vapor, by the contraction of which sun, planets, and moons have 

 been formed, all these bodies must necessarily possess mainly the 

 same constitution. Geology teaches us that the earth once existed in 

 a state of fusion ; and we are compelled to admit that the same state 

 of things has occurred in the other members of our solar system. 

 The amount of cooling which the various heavenly bodies have 

 undergone, in accordance with the laws of radiation of heat, differs 

 greatly, owing mainly to the difference in their masses. Thus, whilst 

 the moon has become cooler than the earth, the temperature of the 

 surface of the sun has not vet sunk below a white heat. 



' 



" Our terrestrial atmosphere, in which now so few elements are 

 "found, must have possessed, when the earth was in a state of fusion, 

 a much more complicated composition, as it then contained all those 

 substances which are volatile at a white heat. The solar atmosphere 

 at this present time possesses a similar constitution. The idea that 

 the sun is an incandescent body is so old that we find it spoken of by 

 the Greek philosophers. When the solar spots were first discovered, 

 Galileo described them as being clouds floating in the gaseous atmos- 

 phere of the sun, appearing to us as dark spots on the bright body of 

 the luminary. He says that if the earth were a self-luminous body, 

 and viewed at a distance, it would present the same phenomena as 

 we see in the sun." Kirchhoff; Researches, etc., p. 24. 



Certain appearances connected with those spots on the sun's sur- 

 face have induced astronomers in general to adopt a different theory 

 of the constitution of the sun from that proposed by Galileo and sup- 

 ported by Kirchhoff. This theory supposes, according to Sir William 

 Herschel, that the centre of the spot reveals a portion of the dark 

 surface of the sun, seen through two overlying openings one 

 formed in a photosphere, or luminous atmosphere, surrounding the 

 dark solid nucleus, and the other in a lower,' opaque, or reflecting 

 atmosphere. The supposition of the existence of such an intensely 

 ignited photosphere surrounding a cold nucleus is, according to 

 Kirchhoff, a physical absurdity. He puts forward his views on this 

 point clearly and forcibly in the following passage : 



" The hypothesis concerning the constitution of the sun which has 

 been thus put forward in order to explain the phenomena of the sun- 

 spots, appears to me to stand in such direct opposition to certain 

 well-established physical laws, that, in my opinion, it is not tenable, 

 even supposing that we were unable to give any other explanation 

 of the sun-spots. This supposed photosphere must, if it exists, radiate 

 heat towards the sun's body as well as from it. Every particle of the 

 upper layer of the lower or opaque atmosphere will therefore be 

 heated to a temperature at least as high as that to which it would be 

 raised if placed on the earth, exposed to the sun's rays, in the focus 

 of a circular mirror whose surface, seen from the focus, is larger than 

 a hemisphere. The less transparent the atmosphere is, the quicker 

 will this temperature be attained, and the smaller will be the distance 

 to which the direct radiation of the photosphere will penetrate into 

 the mass of the atmosphere. What degree soever of opacity the 

 atmosphere may possess, it is certain that in time the heat will be 

 transmitted, partly by radiation, partly by conduction and convection. 



