146 SCIENCE IK SHORT CHAPTERS. 



vapor of moderate density, the spectrum is not a continuous 

 band with its colors gradually blending ; it consists only of 

 certain luminous stripes with blank spaces between them, each 

 particular gas or' vapor showing its own particular set of stripes 

 of certain colors, and always appearing at exactly the same 

 place, so invariably and certainly, that, by means of such 

 luminous stripes, the composition of the gas or vapor may be 

 determined. If, however, the gas be much compressed, the 

 stripes widen as the condensation proceeds ; they may even 

 spread out sufficiently to meet and form a continuous spectrum 

 like that from a solid. Liquids also produce continuous spectra. 



2d. When a luminous solid or liquid, or very dense gas, 

 capable of producing a continuous spectrum, is viewed through 

 an intervening body of other gas or vapor of moderate or 

 small density, fine dark lines cross the spectrum in precisely 

 the same places as the bright stripes would appear if this inter- 

 vening gas or vapor were luminous and seen by itself. 



When the spectroscope is directed to the face of the sun 

 under ordinary circumstances, it presents a brilliant continuous 

 spectrum, striped with a multitude of the dark lines. From 

 this it has been inferred that the luminous face of the sun is 

 that of an incandescent solid or liquid, and that it is surround- 

 ed by the gases and vapors whose bright stripes, when artifi- 

 cially produced, occupy precisely the same places as the dark 

 lines of the solar spectrum. This was the theory of KirchhofE 

 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 con- 

 tinuous spectrum, opened another speculation, far more con- 

 sistent with the other known facts concerning the constitution 

 of the sun viz. that the sun may be a great gaseous orb, blaz- 

 ing at its surface and gradually increasing in density from the 

 surface toward the centre. 



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

 magnesium, iron, chromium, nickel, copper, zinc, strontium, 

 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 ? 



