154 CONSTITUTION OF THE STARS. PABT i. 



incandescent vapour of magnesium, and partly to the 

 intensely heated magnesia formed by the combustion. 

 The properties of this light having been examined and 

 compared with those of the sun by Professors Roscoe 

 and Bunsen, with a view to photographic purposes, they 

 came to the conclusion that f the steady and equable 

 light evolved by magnesium wire, burning in the air, 

 and the immense chemical action thus produced, render 

 this source of light valuable as a simple means of 

 obtaining a given amount of chemical illumination, and 

 that the combustion of this metal constitutes a definite 

 and simple source of light for the purpose of photo- 

 chemical measurement.' 



Bright lines of two different metals sometimes coin- 

 cide with the same black line, that is, they appear to 

 have the same reverse as an iron and a magnesian line, 

 an iron and a nickel line, and some others ; but it is not 

 known whether the coincidence be real or apparent. 



M. Kirchhoff has proved that neither gold, silver, 

 tin, lead, antimony, arsenic, mercury, lithium, cadmium, 

 i and some others are constituents of the sun, because 

 S none of their bright lines are coincident with any of 

 the dark lines of the solar spectrum. This negative 

 discovery does no less honour to M. Kirchlioff than the 

 proof of so many substances being common to the earth 

 and sun. 



Since all incandescent solid and liquid bodies give 

 a continuous spectrum which exhibits no dark lines, M. 

 Kirchhoff conceives that the sun consists of a solid or 

 liquid nucleus, heated to the temperature of the most 

 dazzling whiteness, and that it is surrounded by a lumi- 

 nous gaseous atmosphere of somewhat lower tempera- 

 ture, endowed with the law of exchanges. The spectra 

 of Arcturus, Capella, and many other fixed stars are 

 crossed by dark lines similar to, and often coincident 

 with, the dark lines in the solar spectrum ; therefore, it 



I 



