370 On the Spectra of some Gases under High Pressures. 



not admit that it can be confounded with the first- class spec- 

 trum of this gas, which always retains its tlutings to the moment 

 in which it quite disappears. 



The Ley den jar gave exactly the same spectrum with more 

 brightness in the colours. 



From this abstract of M. Wiillner's memoir it will be seen 

 that the spectrum of one and the sa.ne gas rendered incandescent 

 by the passage of the electric spark varies materially with the 

 temperature and pressure. The spectrum of hydrogen offers 

 four very distinct appearances — that is to say, the spectrum with 

 six groups of green lines, Pliicker's spectrum with three bright 

 lines, the continuous spectrum with two of these three lines (Ha 

 and H/3), and finally the absolutely continuous spectrum pro- 

 duced under high pressures. Oxygen gives also four different 

 spectra; that is to say: — (1) at the lowest limits of pressure, a 

 second-class spectrum consisting of five groups of bright lines in 

 the green and the blue ; (2) at 1 millim. and below, a continuous 

 spectrum consisting of broad bands, especially in the green and 

 the blue; (3) about 10 millims., the second-class spectrum de- 

 scribed by Pliicker; and (4) finally, at high pressures, a conti- 

 nuous spectrum with a large number of bright lines persisting 

 in the more refrangible part. Nitrogen gives two spectra — that 

 is to say, a continuous but fluted spectrum at low pressures, and 

 at high ones a continuous spectrum without fluting and on 

 which are seen to appear a great number of bright lines, first in 

 the green and the blue, and then also in the red. A more pro- 

 found investigation will show whether these spectra are abso- 

 lutely distinct. M. Wullner is led to believe so, and therefore 

 also to assume that the emissive power of incandescent gases for 

 rays of different colours undergoes sudden variations as the 

 pressure of the gases themselves gradually varies. This im- 

 portant point is not adequately cleared up by M. Wiillner's 

 numerous observations. 



On the other hand, M. Wullner has been led by this latter 

 research to modify the ideas which he at first shared with MM. 

 Pliicker and Hittorff, and according to which continuous or first- 

 class spectra corresponded to a lower temperature than spectra 

 with bright lines. It would seem, on the contrary, that conti- 

 nuous spectra correspond to the highest temperatures and pres- 

 sures, which would be in conformity with Dr. Frankland's ob- 

 servations on the spectrum of the hydrogen-flame under high 

 pressures. This, however, is a difficult point to verify ; for it 

 has been seen above that the lustre and the temperature of the 

 electric jet do not vary in a regular manner with the pressure of 

 the gas it traverses, M. Wullner intends to resume these re- 

 searches. 



