1881.] on the Origin and Identify of Spectra, 695 



triplets more refrangible than the quintuple group about wave-length 

 2780. The blue lino wave-length about 4570 is best seen in the spark 

 without a jar when the magnesium electrodes are close together, and 

 the rheotome made to work slowly ; and this and tlie other faint lines 

 of the spark at about -ISSG and 4808 require for their detection a 

 spectroscope in which the loss of light is small. 



On the other hand, some additional lines are seen. Of these, the 

 strong line at wave-length 4481 and the weaker line at 4586 are well 

 known. Another faint line in the blue at wave-length 4808 has been 

 observed by us in the spark, and two diffuse pairs between II and the 

 triplet near L. Two ultra-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, 2789*9 

 (Cornu) make a quadruple group with the very strong pair conspicuous 

 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. 



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* noticed the 

 reversal of the two less refrangible 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. The Cailletet 

 pump is well suited for such experiments. 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 

 h 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 disappear, leaving only the h 

 group very bright, and the magnesium-hydrogen bauds which are de- 

 scribed 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 h group were expanded and re- 

 versed 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 h 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 

 which were reversed was not in the middle. When the jar was used, 

 the pressure could not be carried beyond 10 or 12^- atmospheres, as 

 the resistance became then so great that the spark would not pass 

 across the small distance of about 1 millim. between the electrodes. 



* 'Cumpt. Rcud.' 1871. 



