8 FORMATION OF THE EARTH 



some day know of what the stars and the sun were made and 

 whether the atmosphere of the planets was or was not charged 

 with aqueous vapour, so daring a prophecy would have been 

 regarded as the product of a deranged imagination. Yet such 

 knowledge has been attained, and it is light itself which has 

 furnished us with this information. Everyone knows that if 

 a very narrow ray of white light is allowed to impinge on a 

 triangular crystal prism, perpendicularly to one of its faces, 

 it changes its direction in traversing the prism and, in emerging, 

 spreads out fanwise, the rays as they shade insensibly into one 

 another being of different colours, and the same colours always 

 following on in the same order. Beginning with that part of the 

 fan furthest from the original direction we have the following : 

 Violet, indigo, blue, green, yellow, orange, red. We arrange 

 them in this order instead of the reverse, because the names of 

 the colours then form a word series easily remembered. Violet 

 is the colour with the greatest refractability, red with the least. 

 This fan, whose colours seem to form a magnificent brilliant 

 band when a white screen is placed in its path, is called the solar 

 spectrum. If the ray is sufficiently fine and the prism thick 

 enough for the opening of the fan to be of considerable size, 

 black lines and dark bands are seen in the spectrum. These are 

 the Frauenhofer lines, which justly bear the name of the German 

 physicist who discovered them. On the other hand, the French 

 physicist, Foucault, had pointed out that the spectrum of 

 metals at white heat was not continuous ; that it was composed 

 of lines and of brilliant patches. A little later in Germany, 

 Kirchoff and Bunsen showed that if a beam of continuous 

 white light is made to pass across a dark metallic vapour such 

 as incandescent carbon emits, its spectrum shows dark lines 

 exactly corresponding to the brilliant lines which would be 

 found in the spectrum of the metal emitting the vapour. In 

 other words, from the point of view of luminous intensity, a 

 reversed spectrum of this metal is obtained. Now, on comparing 

 the Frauenhofer lines with the brilliant lines of the spectra 

 of various metals, they were found to be exactly super- 

 imposable, thus indicating the presence of these metals in 

 the solar atmosphere. The study of this atmosphere charged 

 with metallic vapours has been carried very far by the work of 

 the French astronomer, Jannsen, and has demonstrated that 

 all the elements therein contained are found also on the earth. 



