368 PHYSIOGRAPHY. [CMAP. 



Janssen and Mr. Lockyer, whereby these prominences may 

 be examined without waiting for an eclipse. Such examina- 

 tion has shown that the red flames consist, for the most part, 

 of the gas hydrogen (p. T03). Above the region of incan- 

 descent hydrogen, there appears to be an enormous enve- 

 lope of the same gas in a comparatively cool state. It is 

 curious to find that the gas, which forms so large a propor- 

 tion of the water of the earth, should be an important 

 constituent of the sun. One of the chief chemical com- 

 ponents of the river Thames is, in fact, one of the chief 

 components of our central luminary. 



It seems almost incredible that a person situated on the 

 earth should be able to learn anything about the chemical 

 constitution of the sun, which we know to be at least 91 

 millions of miles away. To attempt to subject the sun to 

 any of the ordinary chemical processes of the laboratory is, 

 of course, quite out of the question ; but, within the last 

 twenty years a new method of analysis has been introduced, 

 whereby a great deal may often be learnt about the chemical 

 composition and the physical constitution of an unknown 

 body, by proper examination of the light which is emitted 

 v/hen the body has been heated until it becomes luminous. 



Without entering into a close description of this method, 

 which has been appUed with such signal success to the 

 examination of the sun, it is sufficient to remark that, when 

 a beam of sunshine is allowed to pass through a small 

 aperture in the wall of a dark chamber, and then to 

 traverse a three-sided glass prism, like the drop of a lustre, 

 it does not fall as a spot of white light, but is turned aside 

 from its course, and spreads out into a broad band, which 

 presents all the colours of the rainbow. This coloured 

 ■band is called a spectrum. The course of such a beam of 

 light is exhibited in Fig. 122 ; where A is the slit through 



