96 



Profs. Liveing and Dewar. 



[Jan. 15, 



at 5210, in nitrogen, without being- able to detect C or F either at the 

 same time or by merely varying the discharge by means of a Leyden 

 jar. 



Experiments made in the same way with carbonic oxide instead of 

 nitrogen led to precisely similar results. 



2. Observations on the Spark between Magnesium Points in Hydrogen at 



reduced Pressures. 



A tube, similar to those employed with nitrogen and carbonic oxide, 

 was attached at one end to a Sprengel pump and mercury gauge, and at 

 the other to an apparatus for generating hydrogen. Dry hydrogen was 

 passed through for some time, and the connexion with the hydrogen 

 apparatus closed. On sparking with the hydrogen at the atmospheric 

 pressure, the line at 5210 and its attendant series were visible, and were 

 still visible when a small Leyden jar was used with the induction coil, but 

 disappeared almost entirely when a large Leyden jar was used. When 

 the pressure of the hydrogen was reduced to half an atmosphere, the 

 line at 5210 was seen faintly when a large Leyden jar was used, but 

 not the series of fine lines. When the pressure was reduced to 

 180 millims., the series of fine lines began to show when the large jar 

 was used. By still further reducing the pressure the whole series was 

 permanently visible when the large jar was used; but when the 

 exhaustion was carried still further they grew fainter, and almost dis- 

 appeared. On gradually readmitting hydrogen, the same phenomena 

 recurred in the reverse order. 



3. Observations on the Arc tuith Magnesium and Hydrogen. 



The line at 5210 is not seen in the arc in a lime or carbon crucible 

 when magnesium is dropped in without the introduction of hydrogen. 

 If, however, a gentle stream of hydrogen or of coal gas be led in 

 through a perforation in one of the electrodes, the line at 5210 im- 

 mediately makes its appearance, and, by varying the current, it may 

 be made to appear either bright or reversed. However small the 

 current of hydrogen, be made, the line can be detected as long as the 

 current and the supply of magnesium continues, and disappears very 

 quickly when the current ceases. 



4. Observations on the Flame of Burning Magnesium. 



The line at 5210 may often be seen in the flame of magnesium 

 burning in air, but both it and the series of fine lines which accom- 

 pany it come out with greatly increased brilliance if the burning 

 magnesium be held in a jet of hydrogen, of coal gas, or of steam. 



The experiments above described, with nitrogen and carbonic oxide 

 at reduced pressures, are almost if not quite conclusive against the 

 supposition that the line at 5210 is due merely to the lower tempera- 



