metals, which they termed Caesium and Rubidium. Shortly afterwards 

 our own countryman Mr. Crookes saw in his spectroscope, when examining 

 the residue from burning pyrites, a beautiful green line that was new to 

 him and this turned out to be jiroduced by the salts of a metal till then 

 unknown, but which he was not long in preparing by the cwt. He termed 

 it Thallium. Indium is a fourth element discovered in the same way. 



The following table will give you some idea of the delicacy of this 

 mode of analysis and of the quantities which may easily be detected. 1. 



3,000,000 -t^ ° ' 180,000,000 



2. Lithium, ^^^th part of a millegrarame, or jr^oi^oo*^ ^^^^ °^ ^ S'^''^^'^' 



3. Strontium, j^^th of a millegramme, or ^5^50*^ "^ ^ S^'^^"- *' 

 Calcium, j^— gg-th of a millegramme, or __^_th of a grain. 



We will now pass to the examination of the sun by the spectroscope. 

 I told you that the solar spectrum was crossed by a large number of very 

 fine dark lines. It had long been known that one of these, the D line, 

 corresponded in position with the bright yellow line prodticed by Sodium. 

 Kirchoff and Bunsen discovered that other dark lines corresponded in 

 position to the bright lines found in the spectrum of other elements and 

 upon carefully comparing that of iron, which contains no less than 460 

 bright lines, they found that each one of these was matched by a dark 

 line in the solar spectrum, and the conclusion was irresistibly forced upon 

 them that these dark lines must be caused by iron existing in the sun. 

 But why were the lines dark and not bright 1 They solved this question 

 by observing that the sodium flame was opaque to the light which it itself 

 emitted. Thus if we have a glowing substance producing this particular 

 ray D and place before it a sodium flame we see a black patch in the place 

 of the yellow flame. This is exactly the state of the case in the sun. 

 The body of the sun, which is termed its "photosphere," contains certain 

 glowing elements, and the vapour of these elements, at a much higher 

 temperature, surrounds the photosphere ; the particular rays emitted by 

 the substances in the photosphere are absorbed by this vapour and instead 

 of seeing a bright line we see a dark one. In the case of certain of the 

 stars some of the lines are actually bright. The following bodies have in 

 this way been detected in the sun : — 



