100 SCIENOE IN SHORT CHAPTERS. 



bright stripes, when artificially produced, occupy precisely 

 the same places as the dark lines of the solar spectrum. 

 This was the theory of Kirchoff and others in the early 

 days of spectrum analysis, when it was only known that 

 solids and liquids were capable of producing a continuous 

 spectrum. The important discovery that gases and vapors, 

 if sufficiently condensed, will also produce a continuous 

 spectrum, opened another speculation, far more consistent 

 with the other known facts concerning the constitution of 

 the sun, viz., that the sun maybe a great gaseous orb, blaz- 

 ing at its surface and gradually increasing in density from 

 the surface towards the centre. 



According to this, the metals sodium, calcium, barium, 

 magnesium, iron, chromium, nickel, copper, zinc, stron- 

 tium, cobalt, manganese, aluminium, and titanium, whose 

 vapors, with those of some few other substances, give the 

 dark lines that cross the solar spectrum, should exist neither 

 as solids nor liquids on the solar surface, but as blazing 

 gases. But such blazing gases, according to what I have 

 stated above, should give us bright stripes instead of dark 

 lines. Why, then, are not such bright stripes seen under 

 ordinary circumstances? 



This is easily answered. These blazing gases must, as 

 we proceed from the surface of the sun downwards, become 

 so condensed by the pressure of their own superincumbent 

 strata, as to produce a continuous spectrum of great bril- 

 liancy. With such a background the bright stripes would 

 be confounded and lost to sight. Besides this, the outer 

 film of cooler vapor through which our vision must neces- 

 sarily penetrate before reaching the luminous solar surface, 

 will produce the dark lines exactly where the bright stripes 

 should be, and thus effectually obliterate them ; or, in other 

 words, the intervening non-luminous vapors are opaque to 

 the particular rays of light which the bright vapors of the 

 same substance emits. 



Therefore, according to this theory, if we could sweep 

 away these outside darkening vapors, and screen off the in- 

 ner layers of denser blazing matter which produces the 

 continuous background, we should have a spectrum dis- 



