PHYSICS NINETEENTH CENT.SPECTROSCOPY. 495 



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of our terrestrial elements, he attached the chemical symbol of that 

 element. In this way the 460 bright lines which belong to the spark- 

 spectrum of iron were found to agree with Fraunhofer lines. The 

 conclusion, therefore, was inevitable that iron, as vapour must exist 

 somewhere between the nucleus of the sun and.- -the spectroscope. 

 That the atmosphere of the sun itself was the only pkce where such 

 vapour could exist was apparent from several consideration^.. When 

 the presence in the sun's atmosphere of some of our terrestrial elements 

 had thus been demonstrated, it seemed reasonable to attribute the rest 

 of the Fraunhofer lines to other elementary bodies. Kirchhoff accord- 

 ingly examined the spectra of calcium and magnesium, and he found 

 the most perfect coincidence in the position and intensity of the lines in 

 these cases also. In short, his observations led him to conclude that in 

 the sun's atmosphere the following metals certainly exist, viz., sodium, 

 calcium, barium, strontium, magnesium, iron, nickel, cobalt, chromium, 

 copper, zinc, cadmium, manganese, and hydrogen. The presence of 

 these elements in the sun's atmosphere accounts for a certain number 

 of the Fraunhofer lines, and another considerable number are known 

 to be due to the earth's atmosphere, for this produces an effect analo- 

 gous to the sun's atmosphere. There still remain the greater number 

 of the Fraunhofer lines as yet not identified, and this fact suggests the 

 existence in the sun of many substances with which we are not ac- 

 quainted on the earth. 



These very remarkable results were announced by Kirchhoff in 1861. 

 They harmonized well with the cosmical theories of geologists and 

 astronomers, as the following words of Kirchhoff himself will show : 



" In order to explain the occurrence of the dark lines in the solar 

 spectrum, we must assume that the solar atmosphere encloses a lumi- 

 nous nucleus, producing a- continuous spectrum, the brightness of 

 which exceeds a certain limit. The most probable supposition which 

 can be made respecting the sun's constitution is that it consists of a 

 solid or liquid nucleus heated to a temperature of the brightest white- 

 ness, surrounded by an atmosphere of somewhat lower temperature. 

 This supposition is in accordance with Laplace's celebrated molecular 

 theory respecting the formation of our planetary system. If the matter 

 now concentrated in the several heavenly bodies existed in former 

 times as an extended and continuous mass of vapour, by the contrac- 

 tion 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 yet sunk below 



