8 On Double Spectra. [January, 



shape, which formed the upper portion of the shorter leg of a 

 U-tube ; the long leg of which was about eight feet high. The 

 spectral tube was connected by one side-tube with the apparatus 

 for preparing pure hydrogen, and by a second side-tube with 

 a Geissler's air-pump, by means of which any degree of 

 exhaustion could be obtained, while, by pouring mercury 

 into the long leg of the U-tube, the pressure could be increased 

 up to about three atmospheres. 



Prof. Wullner found that, using the discharge from an 

 induction coil, the spectrum obtained varied essentially with 

 the pressure. With the lowest pressure which could be 

 obtained, the light in the tube is of a splendid green, like a 

 thallium flame, and the spectrum does not contain either the 

 red line or the violet line of Pliicker's spectrum, but consists 

 of six groups of very brilliant green lines (Fi'g. 6, 3). At 

 pressures of one, two, or three millimetres the spectrum 

 is the well-known one consisting of the three lines without 

 any of the green lines ; and upon increasing the pressure 

 still further, another spectrum makes its appearance which 

 contains Ha and ]3, but not H7, and the space between Ha 

 and H/3 is filled up by a number of beautifully shaded bands, 

 which also extend somewhat beyond H/3; this spectrum 

 persists till the pressure rises to about 400 millimetres. It 

 is represented in Fig. 6, 2. Prof. Wullner thinks that these 

 different spectra of hydrogen are to be explained by differ- 

 ences of temperature, and that the spectrum last described is 

 due to a higher temperature than that which produces the 

 spectrum of three lines. 



The results obtained with oxygen were quite similar. At 

 the lowest pressure it gave a spectrum consisting of five 

 groups of fine lines in the green and blue ; at pressures of 

 about one millimetre a second spectrum of broad bands ; and 

 at higher pressures a third, consisting of a great number of 

 fine lines. 



Nitrogen, on the other hand, only gave the two spectra 

 already described. 



The same memoir of Prof. Wullner contains a description 

 of two different aluminium spectra. Both were obtained by 

 letting the spark strike between wires of aluminium, but the 

 spectrum varied with the distance between the electrodes. 

 With a distance of about two millimetres the spectrum 

 (Fig. 7, 2) consists of four green splendidly shaded bands, 

 brightest on the side towards the blue, and each traversed by 

 fine bright lines. When, however, the spark distance exceeded 

 ten millimetres, a quite different spectrum was obtained, 

 which is no doubt produced at a higher temperature. This 



