THE CHEMISTRY OF THE STARS. 169 



Now do these chemical elements exist indiscriminately in all the 

 celestial bodies, so that practically, from a chemical point of view, the 

 bodies appear to ns of similar chemical constitution ? No ; it is not so. 



From the spectra of those stars which resemble the sun, in that they 

 consist of an interior nucleus surrounded by an atmosphere which 

 absorbs the light of the nucleus, and which, therefore, we study by 

 means of this absorption, it is to be gathered that the atmospheres of 

 some stars are chiefly gaseous — i. e., consisting of elements we recog- 

 nize as gases here — of others chiefly metallic, of others again mainly 

 composed of carbon or compounds of carbon. 



Here, then, we have spectroscopically revealed the fact that there is 

 considerable variation in the chemical constituents which build up the 

 stellar atmospheres. 



This, though a general, is still an isolated statement. Can we con- 

 nect it with another? One of the laws formulated by Kirchhoff in the 

 infancy of spectroscopic inquiry has to do with the kind of radiation 

 given out by bodies at different temperatures. A poker placed in a 

 fire first becomes red and, as it gets hotter, white hot. Examined in 

 a spectroscope, we find that the red condition comes from the absence 

 of blue light; that the white condition comes from the gradual addition 

 of blue as the temperature increases. 



The law affirms that the hotter a mass of matter is the farther its 

 spectrum extends into the ultraviolet. 



Hence the hotter a star is the farther does its complete or continuous 

 spectrum lengthen out toward the ultraviolet and the less is it absorbed 

 by cooler vapors in its atmosphere. 



Now, to deal with three of the main groups of stars, we find the fol- 

 lowing very general result: 



Gaseous stars Longest spectrum. 



Metallic stars Medium spectrum. 



Carbou stars Shortest spectrum. 



We have now associated two different series of phenomena, and we 

 are enabled to make the following statement: 



Gaseous stars Highest temperature. 



Metallic stars Medium temperature. 



Carbon stars Lowest temperature. 



Hence the differences in apparent chemical constitutions are associ- 

 ated with differences of temperature. 



Can we associate with the two to which I have already called atten- 

 tion still a third class of facts'? Laboratory work enables us to do this. 

 When I began my inquiries the idea was, one gas or vapor one spec- 

 trum. We now know that this is not true; the systems of bright lines 

 given out by radiating substances change with the temperature. 



We can get the spectrum of a well-known compound substance — say 

 carbonic oxide; it is one special to the compound; we increase the 



