154 



Drs. Plucker and Hittorf on the Spectra of [Mar. 3, 



examined, or, if it be volatilized or already in the state of gas, to exhibit its 

 characteristic lines. In most cases it is only the electric current that is 

 fitted to produce these lines ; and the current furnished by a powerful in- 

 duction 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 vola- 

 tilized 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 con- 

 sisting 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 introduced into the vessel, 

 which is then exhausted as highly as possible, and the substance is heated 

 by a lamp at the time of the observation. In the third case the electric 

 current is employed at the same time for volatilizing the body and render- 

 ing 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 continuous 

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

 trodes 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 inter- 

 posed gas, as well as those due to the substance 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 sufiice for the other bodies 

 examined. 



On sending through a capillary tube containing nitrogen, at a pressure of 

 from 40 to 80 millimetres, the direct discharge of a powerful Ruhmkorff's 

 coil, a spectrum is obtained consisting, both in its more and in its less re- 

 frangible 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 arrange- 



