362 CLASSICS OF MODERN SCIENCE 

 Now, with regard to the metallic substances which we find, we deal 

 chiefly with calcium, strontium, iron, and magnesium. Others are 

 not absolutely absent, but their percentage quantity is so small that 

 they are negligible in a general statement. 



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 us 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 addi- 

 tion 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 continu- 

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

 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. 



Carbon stars Shortest spectrum. 



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

 are enabled to make the following statement : 



