82 M. E. Wiedemann's Investigations on 



here intimated, though I hope soon to be able to communicate 

 experimental data for the solution of the above-mentioned 

 problem. 



If we compare the line spectra of chemically similar ele- 

 ments, such as potassium, sodium, calcium, and rubidium, 

 they show (as was first remarked by Lecoq de Boisbaudran) 

 the same groups of lines, only in different places in the spec- 

 trum. Hence they are composed of the same harmonic vibra- 

 tions of different fundamentals ; therefore the configurations 

 of the envelopes enclosing the material atoms, or the forces 

 acting upon these, must be similar. A displacement of the 

 lines corresponding to one another in the spectra in question 

 from the red towards the violet signifies, cceteris paribus, an 

 increase of the attractive force of the atoms upon the aether 

 envelopes enclosing them. 



Band Spectra. — If we consider gases which are composed 

 not of single atoms but of molecules, we shall then often have 

 to investigate not the spectra of emission, but of absorption, 

 since at the temperatures at which those gases begin to be 

 luminous their molecules are already broken up. Whether 

 the molecules be composed of homogeneous or heterogeneous 

 atoms, the spectra will always be conditioned by the rotatory 

 or oscillatory motions either of the entire molecules or of the 

 atoms in them or the aether envelopes enclosing the latter ; 

 hence they will possess a corresponding character. Thus, 

 with simple gases at low temperatures the so-called band 

 spectra appear, quite analogously to which the spectra of com- 

 binations are composed. Both consist of broad bands of light, 

 which upon closer examination prove to be formed of bright 

 and dark lines. So it is with nitrogen, and with carbonic 

 acid ; just so, as I have repeatedly convinced myself, do the 

 haloid compounds of the metals of the alkaline earths, and 

 similarly also those of mercury (conf. the experiments of Mr. 

 Pierce in a memoir that will shortly appear). 



We will inquire first whether these spectra are to be attri- 

 buted to the rotatory motions of the entire molecules*. 



The fundamental vibration of the rotatory motion we can 

 calculate approximately by means of the mechanical theory of 

 gases. Let m be the mass of a molecule, u the velocity of 



* A similar view was conjecturally expressed by Helmholtz in a paper 

 by J. Moser. Lockyer likewise refers the different spectra of the same 

 body to differently complicated complexes of atoms. The question 

 whether to the same elements only one (a line spectrum) or a plurality 

 of spectra (of lines and bands) can belong, can be decided in favour of 

 the latter view by "Wullner's last experiments, as well as by Lockyer's 

 investigations on the absorption-spectra of differently heated vapours. 



