1 87 1.] On Double Spectra. 7 



characteristics of those produced by compounds and by 

 elements. It remains for future experiments to confirm or 

 modify the indication thus given of the compound nature of 

 nitrogen. Similar results were obtained by Pliicker with 

 sulphur. 



In order to experiment with sulphur, a tube of difficultly 

 fusible glass was employed. Sulphur was introduced into 

 the tube, which was then completely exhausted. When the 

 narrow part of the tube is gently warmed by a spirit-lamp, 

 and the platinum wires are connected with the induction 

 coil, a spectrum of the first order is obtained. It consists of 

 thirty-seven well-defined bands extending from the red into 

 the extreme blue. Seven lie between the solar lines c and d, 

 eighteen between D and F, and eleven between f and G. On 

 heating the tube still more a quite different set of bright lines 

 makes its appearance, and on introducing a Leyden jar into 

 the circuit the second spectrum becomes fully established 

 and no trace of the spectrum of bands remains. This second 

 spectrum consists entirely of sharply defined bright lines — 

 two red lines are especially noticeable, each of them triple. 



It is thus established that certain gases may, under altered 

 circumstances, vibrate in an entirely different manner, and 

 Pliicker believes that the necessary difference of circum- 

 stance is simply difference of temperature, the spectra of 

 the first order belonging always to the lower temperature. 

 The Leyden jar increases the temperature of the gas, for it 

 necessitates the accumulation of a larger quantity of electri- 

 city preparatory to each discharge, so that the temperature 

 of the spark with the Leyden jar is much higher than that 

 of the simple discharge. Thus we see that, on heating the 

 sulphur-tube and employing the Leyden jar, the low tem- 

 perature spectrum gives way to the high temperature 

 spectrum. 



The spectrum obtained from an ordinary vacuum-tube 

 containing hydrogen (under a pressure of 5 to 10 millimetres), 

 consists of three lines only — Ha, coincident with Fraunhofer's 

 line c in the red ; H/3, coincident with F in the blue; and Hy, 

 nearly coincident with G in the violet. 



Pliicker, who first observed this spectrum (Fig. 6), 

 described, in the paper already referred to, a second spectrum 

 of hydrogen, corresponding to a lower temperature. This 

 observation has been abundantly confirmed by important 

 experiments, made since the date of Pliicker's paper, by 

 Prof. Wiillner, of Bonn ; and his results are so remarkable 

 that it will be well to describe them somewhat at length. 

 His apparatus consisted of a vacuum-tube of the ordinary 



