CHAPTER XXVII. 



TESTS SUPPLIED BY ARC PHENOMENA. 



1. Line Absorption in the Naked Arc. 



I DO not know whether there is on record any observation of 

 line absorption as it is now called, that is of a bright line having 

 a dark absorption line in its centre, earlier than that published 

 in 1872. 1 This referred to an observation made by Dr. Frank- 

 land and myself in 1868, which was first demonstrated at the 

 Royal Institution in May 1869. In our researches on sodium 

 we had observed that each of the bright D lines under certain 

 circumstances was traversed by an absorption line sometimes as 

 delicate as a spider's web, sometimes thickening extremely, 

 thereby putting on, in fact, the appearances presented by the F 

 and C lines of hydrogen bright at the sun's limb; a condition 

 of things predicted I believe by Mr. Johnston Stoney before it 

 was actually observed. 



Very shortly after the beginning of 1869 I first began the 

 study of the lines seen in the electric arc, an image of the arc 

 being thrown on the slit. The beauty of these line reversals is 

 so great that they have not yet lost their charm for me ; even 

 now it is a delight to look at them and watch their changes 

 under the various conditions. 



The origin of the phenomena may be thus stated : 



When a mass of metal in the electric arc, in air, is driven into 

 1 Phil Trans. 1873, p. 253. 



