1881.] on the Orifjhi and Identitij of Spectra. GDI 



did not immediately disappear, but the cxjmnsion of the magnesium lines 

 and the increase of the continuous spectrum seemed to overpower them. 



When nitrogen was substituted for hydrogen, the strongest lines 

 of the green flutings were seen when the spark without jar was first 

 passed at atmospheric pressure, probably from hydrogen occluded, 

 as it usually is, in the magnesium electrodes. As the pressure was 

 increased they speedily disappeared entirely, and were not again seen 

 either at high or low pressures. 



With carbonic oxide the same thing occurred as with nitrogen ; 

 but in this gas the flutings due to the oxide of magnesium (wave- 

 length 4930 to 5000) were, for a time, very well seen. 



Fig. 4, Plate III., shows more completely than we have given it 

 before the general character of the magnesium-hydrogen spectrum, 

 which consists of two sets of flutings closely resembling in character 

 the hydrocarbon flutings, each fluting consisting of a multitude of 

 fine lines closely set on the less refrangible side, and becoming 

 wider apart and weaker towards the more refrangible side, but ex- 

 tending under favourable circumstances much farther than is shown 

 in the figure. The set in the green is the stronger, and it was 

 to this that our former observations were confined. It has two 

 flutings, one beginning at about wave-length 5210 and the other 

 close to 6, on its more refrangible side. The other set consists of 

 three principal flutings, of which the first begins at about wave- 

 length 5618, the next at about wave-length 5566, and the third 

 begins with three strong lines at about the wave-lengths 5513, 5512, 

 5511. Both sets are very well seen when a magnesium wire is 

 burnt in the edge of a hydrogen flame, and in the arc in a crucible of 

 magnesia when a gentle current of hydrogen is led into it. There is 

 also a pair of bands in the blue beginning at about the wave-lengths 

 4850, 4802. 



Mr. Lockyer states {Joe. cii.) that none of the lines of magnesium, 

 special to the flame spectrum, are visible in the spectrum of the 

 chloride, either when flame or spark is employed. But we find that 

 when the spark is taken between platinum points, from a solution of 

 the chloride of magnesium, in a tube such as those used by Delachanal 

 and Mermet, the line at wave-length 5210 can frequently be seen in it 

 when the tube is filled with air, and that if the tube be filled with 

 hydrogen the green flutings of magnesium-hydrogen are persistent 

 and strong. 



Repeated observations have confirmed our previous statements as 

 to the facility with which the magnesium-hydrogen spectrum can be 

 produced in the arc by the help of a current of the gas. In a 

 magnesia crucible, by regulating the current of hydrogen, the flutings 

 can be easily obtained either bright or reversed. 



The variations in the spectrum of magnesium, and the conditions 

 under which it is observed, throw additional light on the question of 

 the emissive power for radiation of short wave-lengths of substances 



Vol. IX. (No. 74.) 3 b 



