1881.] Investigations on the Spectrum of Magnesium. 19o 



observed by ns in the spark, and two diffuse pairs between H and the 

 triplet near L. Two nltra- violet lines at wave-lengths 2934-9, 2926*7 

 (Cornu) are near, but not identical with, two lines of the arc above 

 mentioned ; and two more lines at wave-lengths 2797*1, 2 789 "9 (Cornu) 

 make a quadruple group with the very strong pair which are con- 

 spicuous in the arc in this region. The spectrum of the spark ends, 

 so far as we have observed, with the quintuple group (2782 — 2776) 

 already described in the arc. The lines of this spectrum are given in 

 fig. 3. 



When a Leyden jar is used with the coil, some of the lines are 

 reversed. This is notably the case with the triplet near L, the line at 

 wave-length 2850, and those at 2801 and 2794. Cornu (' 4 Compt. 

 Rend.," 1871) noticed the reversal of the less refrangible two lines of 

 the triplet near L under these circumstances. This effect is very 

 much increased by increasing the pressure of the gas in which the 

 spark is taken. For the purpose of observing the influence of increased 

 pressures, we have used a Cailletet pump and glass tubes similar 

 to those employed in the liquefaction of gases by means of such a pump, 

 but with an expansion of the upper part in which were magnesium 

 electrodes attached to platinum wires sealed into the glass. The tube 

 having been filled with gas at the atmospheric pressure, was sealed at 

 its upper end, while the lower end dipped into mercury contained in 

 the iron bottle of the Cailletet pump, and the gas was afterwards com- 

 pressed by driving more or less mercury into the tube. The gases 

 used were hydrogen, nitrogen, and carbonic oxide ; and the image of 

 the spark was thrown on to the slit of the spectroscope by a lens. In 

 hydrogen, when no Leyden jar was used, the brightness of the yellow 

 and of the blue lines of magnesium, except at first that at wave-length 

 4570, diminished as the pressure increased ; while, on the other hand, 

 the b group was decidedly stronger at the higher pressure. The 

 pressure was carried up to 20 atmospheres, and then the magnesium 

 lines in the blue and below almost or entirely disappeared, leaving only 

 the b group very bright, and the magnesium-hydrogen bands which 

 are described below; even the hydrogen lines F and C were not 

 visible. When a jar was used, the magnesium lines expanded as the 

 pressure was increased ; all three lines of the b group were expanded 

 and reversed at a pressure of 5 atmospheres ; the yellow line, wave- 

 length 5528, was also expanded but not reversed ; and the line at 4481 

 became a broad, very diffuse band, but the line at wave-length 4570 

 was but very little expanded. The expansion both of the b group and 

 of the yellow line seemed to be greater on the less refrangible than on 

 the more refrangible side of each line, so that the black line in those 



vol. 27, p. 353), but the wave-length is there given, through an error in taking out the 

 ordinate of the curve of interpolation, as 4797 instead of 4807. Another measure 

 has given the wave-length 4808. 



