1831.] on the Teacliinjs of Modern Spectroscopy. 499 



may reuder the bands more wide and indistinct at tlieir edges, but its 

 influence is more of a nature which in gas spectra is sometimes 

 observed at high pressures when the lines widen, and does not con- 

 sist of an alteration in type. Though in a solid or liquid body the 

 molecules are much nearer together, tliey are less mobile ; and hence 

 the nuiiiber of actual collisions need not be necessarily much in- 

 creased. The fact that a crystal may show a ditrerence in the absorp- 

 tion sjicctrum according as the vibrations of the transmitted light take 

 place along or across tlie axis, shows, I tliink, that mutual impacts 

 cannot much atfect tlie vibrations, but that each molecule, at least in 

 a crystal, must be ke2)t pretty well in its place. 



We have divided spectra into three types, but in all attempts at 

 classitication we are met by the same ditBculty. The boundaries 

 between the different types are not in all cases very well mai'ked. 

 Every one will be able to distinguish a well-defined band- spectrum 

 from a line-spectrum, but there are spectra taking up intermediate 

 positions both between the line- and band-spectra and between band- 

 spectra and continuous spectra. With regard to these it may be 

 difficult to tell to which type the spectrum really belongs. It may- 

 happen that a change of spectrum takes place, the spectrum retaining 

 its type ; but in these cases, as a rule, the more complex molecule 

 will have a spectrum approaching the lower type, although it may 

 not actually belong to that lower type. To be joerfectly general, 

 we may say that a combination of atoms always produces an alteration 

 in the spectrum in the direction of the change from the line-spectrum, 

 through the band-spectrum to the discontinuous spectrum. 



If we accept the now generally received opinion as to the cause of 

 the diiferent types of spectra, we may obtain information on molecular 

 arrangement and complexity where our ordinary methods fail. At 

 high temperatures, or under much diminished pressure, measures of 

 density become difficult or impossible ; and it is just in these cases 

 that the spectroscope furnishes us with the most valuable information. 

 If we find three spectra of nitrogen and the same number for oxygen, 

 we must accept the verdict, and conclude that these gases can exist in 

 three different allotropic states. 



Amongst the remarkable phenomena observed in vacuum tubes, 

 perhaps not the least curious is the spectrum observed at the negative 

 pole, which in several cases is only observed tliere, and under ordinary 

 circumstances in no other part of the tube. Both oxygen and nitrogen 

 have a sjiectrum whicli is generally confined to the negative glow. 

 Some years ago I tried to prove that also in these cases we have only 

 to deal with a special modification of the gases which, curiously 

 enough, only exists near the negative pole, and is broken up and 

 decomposed in every other part of the tube. The experiments I then 

 made seem to me to prove the point conclusively. After a current of 

 electricity had passed through the tube for some time in one direction, 

 the current was suddenly reversed ; the negative pole now became 

 positive, but the spectrum still was visible for some time in its neigh- 



