326 Professor H. E. Roscoe [March 1, 



detailed information concerning it. The spectrum of the new metal 

 consists of two splendid red lines situated beyond the red line K a in 

 the ultra red portion of the solar spectrum. Hence I propose to call 

 the new metal * Rubidium* " 



That this same method of investigation can be extended to all the 

 metallic elements is more than probable, for Kirchhoff writes — " I have 

 assured myself that even the metals of the rarest earths, as yttrium, 

 erbium, and terbium, can be most quickly and certainly determined 

 by help of the spectrum analytical method." 



Experiments are being carried on with the view of making this 

 mode of examination practically applicable to all metals. 



To turn, now, to the second, and, if possible, to the more inter- 

 esting part of the subject, namely, the conclusions drawn from these 

 observations respecting the composition of the sun's atmosphere. The 

 solar spectrum invariably contains a large number of dark lines, or 

 spaces, or shadows. These have been called Fraunhofer's Lines, from 

 the name of their discoverer. They show us that in the sun's light 

 certain kind of rays are wanting ; and as these lines are always present, 

 exactly in the same position, we see that certain kinds of rays are 

 always absent in solar light. There are many thousands of these lines 

 in the whole length of the spectrum. Only a few have been, as yet, 

 mapped and named. 



What is the cause of these constant dark lines ? And we must 

 remember that it is in sunlight alone that these particular lines occur ; 

 in the light of the fixed stars, as well as in artificial lights, other lines 

 are found. I£ is the discovery of this cause by Kirchhoff which gives 

 the subject such peculiar interest, as it enables us to draw conclusions 

 respecting the composition of the sun's atmosphere. The points of 

 the case are put as concisely as possible under the following heads : — 



1. The solar spectrum invariably contains certain fixed dark lines, 

 called Fraunhofer's Lines. 



2. The spectra produced by the luminous vapour of all metals 

 contain certain fixed bright lines, invariable, and distinct for each 

 metal. 



3. All and each of the bright lines thus produced by certain 

 metals — viz. sodium, potassium, magnesium, and iron — are found to 

 coincide exactly with certain of the dark lines of the solar spectrum. 



4. Hence there must be some connection between the bright lines 

 of the metal, and the dark solar lines. 



5. The connection is as follows : — Each of the dark fixed lines in 

 the solar spectrum is caused by the presence in the sun's atmosphere 

 of the luminous vapour of that metal which gives the coincident bright 

 line. 



By taking a special case we may more easily understand the matter. 

 Let us examine the question why it is to be concluded that Sodium 

 occurs in the sun'^ atmosphere? In the following sentences the 

 reasoning on this subject is rendered clear : — 



