Drs. Plucker and Hittorf on the Spectra of Ignited Gases. 65 



the current furnished by a powerful induction coil was what the 

 authors generally employed. 



In the application of the current, different cases may arise. The 

 body to be examined may be either in the state of gas, or capable 

 of being volatilized at a moderate temperature, such as glass will bear 

 without softening, or its volatilization may require a temperature still 

 higher. 



In the first two cases the body is enclosed in a blown-glass vessel 

 consisting of two bulbs, with platinum wires for electrodes, connected 

 by a capillary tube. In the case of a gas, the vessel is exhausted by 

 means of Geissler's exhauster, and filled with the gas at a suitable 

 tension. In the case of a solid easily volatilized, a portion is intro- 

 duced into the vessel, which is then exhausted as highly as possible, 

 and the substance is heated by a lamp at the time of the observa- 

 tion. In the third case the electric current is employed at the same 

 time for volatilizing the body and rendering its vapour luminous. 

 If the body be a conductor, the electrodes are formed of it ; but the 

 spectrum observed exhibits not only the lines due to the body to be 

 examined, but also those which depend on the interposed gas. This 

 inconvenience is partly remedied by using hydrogen for the interposed 

 gas, as its spectrum under these circumstances approaches to a con- 

 tinuous one. If the body to be examined be a non-conductor, the 

 metallic electrodes are covered with it. In this case the spectrum 

 observed contains the lines due to the metal of which the electrodes are 

 formed, and to the interposed gas, as well as those due to the sub- 

 stance to be examined. 



Among the substances examined, the authors commence with 

 nitrogen, which first revealed to them the existence of two spectra 

 belonging to the same substance. The phenomena presented by 

 nitrogen are described in detail, which permits a shorter description 

 to suffice for the other bodies examined. 



On sending through a capillary tube containing nitrogen, at a pres- 

 sure of from 40 to 80 millimetres, the direct discharge of a powerful 

 RuhmkorfFs coil, a spectrum is obtained consisting, both in its more 

 and in its less refrangible part, of a series of bright shaded bands : 

 the middle part of the spectrum is usually less marked. In each of 

 the two parts referred to, the bands are formed on the same type ; 

 but the type in the less refrangible part of the spectrum is quite 

 different from that in the more refrangible. In the latter case the 

 bands have a channeled appearance, an effect which is produced by 

 a shading, the intensity of which decreases from the more to the less 

 refracted part of each band. In a sufficiently pure and magnified 

 spectrum, a small bright line is observed between the neighbouring 

 channels, and the shading is resolved into dark lines, which are nearly 

 equidistant, while their darkness decreases towards the least refracted 

 limit of each band. With a similar power the bands in the less 

 refrangible part of the spectrum are also seen to be traversed by fine 

 dark lines, the arrangement of which, however, while similar for 

 the different bands, is quite different from that observed in the chan- 

 neled spaces belonging to the more refrangible region. 



Phil. Mag. S. 4. Vol. 28. No. 186. July 1864. F 



