The Romance of the Heavens 23 



Light from any substance which has been made incandescent may 

 be observed with the spectroscope in the same way, and each ele- 

 ment can be thus separated. It is found that each substance (in 

 the same conditions of pressure, etc.) gives a constant spectrum 

 of its own. Each metal displays its own distinctive colour. It is 

 obvious, therefore, that the spectrum provides the means for 

 identifying a particular substance. It was by this method that 

 we discovered in the sun the presence of such well-known elements 

 as sodium, iron, copper, zinc, and magnesium. 



Every chemical element known, then, has a distinctive spec- 

 trum of its own when it is raised to incandescence, and this dis- 

 tinctive spectrum is as reliable a means of identification for the 

 element as a human face is for its owner. Whether it is a sub- 

 stance glowing in the laboratory or in a remote star makes no 

 difference to the spectroscope; if the light of any substance 

 reaches it, that substance will be recognised and identified by the 

 characteristic set of waves. 



The spectrum of a glowing mass of gas will consist in a 

 number of bright lines of various colours, and at various intervals ; 

 corresponding to each kind of gas, there will be a peculiar and dis- 

 tinctive arrangement of bright lines. But if the light from such 

 a mass of glowing gas be made to pass through a cool mass of the 

 same gas it will be found that dark lines replace the bright lines 

 in the spectrum, the reason for this being that the cool gas absorbs 

 the rays of light emitted by the hot gas. Experiments of this 

 kind enable us to reach the important general statement that 

 every gas, when cold, absorbs the same rays of light which it 

 emits when hot. 



Crossing the solar spectrum are hundreds and hundreds of 

 dark lines. These could not at first be explained, because this 

 fact of discriminative absorption was not known. We under- 

 stand now. The sun's white light comes from the photosphere, 

 but between us and the photosphere there is, as we have seen, 

 another solar envelope of relatively cooler vapours the reversing 



