THE SI'ECTRA OF STARS AND NEBULA. 



15 



may represent the products of the dissociation of the ordinary 

 chemical elements under the influence of solar temperatures, but 

 the circumstances that nearly all the stronger lines, and many 

 of the faint ones, have been reproduced under laboratory 

 conditions renders this very improbable. 



In the present state of our knowledge of spectroscopy, we are 

 certainly not entitled to conclude that any unknown forms of 

 matter are represented by the unidentified Fraunhofer lines. 

 The only strong indication of elements not known on the 

 earth is given by the corona, and the discovery of terrestrial 

 helium encourages the hope that even these may yet be run to 

 earth. 



Another side of this question calls for a few remarks. Many 

 of the known elements have not yet been recognized in the sun, 

 and some explanation of their seeming absence is called for if 

 we are to suppose the sun and earth to be composed of the same 

 materials. Some of the non-metallic elements are especially 

 notable among the missing elements, but it is a common 

 experience of the laboratory that such elements often fail to 

 show their spectra when they are admixed with metallic vapours. 

 Also, the Count de Gramont has shown that in the case of 

 several of the missing elements, non-metallic and metallic, the 

 most sensitive lines are situated in the ultra-violet, in a part of 

 the solar spectrum which is cut off by the absorption of our own 

 atmosphere. Another indication that we must not too hastily 

 conclude that elements not represented by Fraunhofer lines are 

 absent from the sun is afforded by the detection of lithium in 

 the spectra of sun-spots, where the physical conditions are 

 apparently more favourable for the exhibition of this element. 

 Further, it is quite reasonable to suppose that some of the 

 heavier metals which fail to give spectroscopic indications of 

 their presence may exist in the interior of the sun, where our 

 spectroscopes are unable to penetrate. 



The outcome of the spectroscopic analysis of the sun, therefore, 

 is to indicate that although there is no complete proof of absolute 

 identity, we should not be justified in supposing that there is 

 any material difference in the composition of the sun and that 

 of the earth. As Eowland expressed it, if the earth were heated 

 to the temperature of the sun it would probably show the same 

 spectrum. 



The sun has been dealt with at some length, because it may 

 be regarded as a typical specimen of the thousands of stars 

 which present us with spectra which cannot be distinguished 

 from that of the sun itself, when sunlight is taken as a whole, 



