694 Professor Dewar [June 10, 



lines to tlie inch, indicate a wave-length 2852 nearly, whicli is still 

 closer to the half of 5710. 



When metallic magnesium is dropped into a crucible of magnesia 

 or lime through which the arc is passing, the electric current seems 

 sometimes to be conducted chiefly or entirely by the vaporised metal, 

 so that the lines of other metals almost or wholly disappear ; but the 

 line at wave-length 3278 does not in such cases appear, though the 

 other magnesium lines are very strongly developed. The line at 

 wave-length 2850 is often, under such circumstances, enormously 

 expanded and reversed, those at wave-lengths 2801, 2794, and the 

 alternate diffuse triplets, including those near L and near S, much 

 expanded and reversed, and the group of five lines (2776-2782) 

 sometimes reversed. 



When the arc of a Siemens machine is taken in a magnesia 

 crucible, the strong line of the flame spectrum, wave-length 4570, is 

 well seen sharply defined ; it comes out strongly and a little expanded 

 on dropping in a fragment of magnesium. When a gentle stream of 

 hydrogen is led in through a hollow pole, this line is frequently 

 reversed as a sharp black line on a continuous background. From 

 comparing the position of this line with those of the titanium lines 

 in its neighbourhood, produced by putting some titanic oxide into the 

 crucible, we have little doubt that it is identical with the solar line 

 4570-9 of Angstrom. 



When the arc is taken in a crucible into which the air has access, 

 it may be assumed that the atmosphere about the arc is a mixture of 

 nitrogen and carbonic oxide. When a stream of hydrogen is passed, 

 either through a perforated pole or by a separate opening, into the 

 crucible, the general effect is to shorten the length to which the arc 

 can be drawn out, increase the relative intensity of the continuous 

 spectrum, and diminish the intensity of the metallic lines. Thus, 

 with a very gentle stream of hydrogen in the magnesia crucible, most 

 of the metallic lines, except the strongest and those of magnesium, 

 disappear. Those lines which remain are sometimes reversed ; those 

 at wave-length 2850 and the triplet near L being always so. With a 

 stronger stream the lines of magnesium also disappear, the h triplet 

 being the last in that neighbourhood to go, and h^ and h.^ remaining 

 after ht had disappeared. 



Chlorine seems to have an opposite effect to hydrogen, generally 

 intensifying the metallic lines, at least those of the less volatile 

 metals, but it does not sensibly affect the spectrum of magnesium. 

 Nitrous oxide produces no marked effect; coal-gas acts much as 

 hydrogen. 



S;pectrum of the Magnesium Sparh, in Gases under High Pressures. 



In the spark of an induction coil taken between magnesium points 

 in air we get all the lines seen in the arc except two blue lines at 

 wave-lengths 4350 and 4166, three lines above U, and the series of 



