NATURAL PHILOSOPHY. - 169 



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sition is rendered still less probable by the fact that these lines do 

 not appreciably alter when the sun approaches the horizon. " It does 

 not, on the other hand, seem at all unlikely, owing to the high tem- 

 perature which we must suppose the sun's atmosphere to possess, 

 that such vapors should be present in it. Hence the observations of 

 the solar spectrum appear to me to prove the presence of iron vapor 

 in the solar atmosphere with as great a degree of certainty as we can 

 attain in any question of natural science." KirchJiqff' ; Researches, 

 etc., p. 20. 



This statement is not one jot more positive than the facts warrant. 

 For to what does any evidence in natural science amount beyond 

 the expression of a probability ? A mineral sent to us from New 

 Zealand is examined by our chemical tests, of which we apply a cer- 

 tain number, and we say these show us that the mineral contains 

 iron, and no one doubts that our conclusion is correct. Have we, 

 however, in tins case proof positive that the body really is iron ? 

 May it not turn out to be a substance which in these respects resem- 

 bles, but in other respects differs from, the body which we designate 

 as iron ? Surely. All we can say is, that in each of the many com- 

 parisons which we have made the properties of the two bodies prove 

 identical ; and it is solely this identity of the properties which we 

 express when we call both of them iron. Exactly the same reasoning 

 applies to the case of the existence of these metals in the sun. Of 

 course the metals present there, causing these dark lines, may not be 

 identical with those which we have on earth ; but the evidence of 

 their being the same is as strong and cogent as that which is brought 

 to bear upon any other question of natural science, the truth of 

 which is generally admitted. 



We do not think we can give our readers a more clear and suc- 

 cinct account of the development of this great discovery than by 

 quoting from Kirchhoff 's admirable Memoir the following passage : 



"As soon as the presence of one terrestrial element in the solar 

 atmosphere was thus determined, and thereby the existence of a 

 large number of Fraunhofer's lines explained, it seemed reasonable 

 to suppose that other terrestrial bodies occur there, and that, by 

 exerting their absorptive power, they may cause the production of 

 other Fraunhofer's lines. For it is very probable that elementary 

 bodies which occur in large quantities on the earth, and are likewise 

 distinguished by special bright lines in their spectra, will, like iron, 

 be visible in the solar atmosphere. This is found to be the case with 

 calcium, magnesium, and sodium. The number of bright lines in the 

 spectrum of each of these metals is indeed small, but those lines, as 

 well as the dark lines in the solar spectrum with which they coincide, 

 are so uncommonly distinct that the coincidence can be observed 

 with great accuracy. In addition to this, the circumstance that 

 these lines occur in groups renders the observation of the coincidence 

 of these spectra more exact than is the case with those composed of 

 single lines. The lines produced by chromium, also, form a very 

 characteristic group, winch likewise coincides with a remarkable 

 group of Fraunhofer's lines ; hence I believe that I ani justified in 

 affirming the presence of chromium in the solar atmosphere. It 

 appeared of great interest to determine whether the solar atmosphere 

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