452 '" ' ' BEPORT — 1880. 



observer. A widening of the lines may also he produced hj increase of pressure, 

 because it diminishes the free path of the molecules, and the disturbances of the 

 ether arising from collisions become more important than vibrations arising from 

 the regular vibrations of the atoms. 



Band spectra, or channelled space spectra, more readily occur in the case of 

 bodies which are not very readily subject to chemical actions, or, according to 

 Professors Liveing and Dewar, in the case of cooler vapours near the point of 

 liquefaction. 



The effects of change of temperature on the character of spectra is very well 

 illustrated by an experiment of M. Wiedemann with mixtures of mercury with 

 hydrogen or nitrogen in a Geissler's tube. At the ordinary temperature of the air 

 the spectrum of hydrogen or nitrogen was obtained alone ; but on heating the tube 

 in an air-bath the lines of mercury appeared and became brighter as the tempera- 

 ture rose, and at the same time the hydrogen lines disappeared in the -wader portion 

 of the tube and at the electrodes. The hydrogen or nitrogen lines disappeared first 

 from the positive electrode and in the luminous tuft, and as the temperature rose 

 disappeared altogether. With nitrogen in a particular e.xperiment, up to 100° C, 

 the nitrogen lines were seen throughout the tube, but from 100° to 230° the nitrogen 

 lines appear towards the negative pole, and the mercury lines are less bright at the 

 negative than at the positive pole, while at about 230° C. no nitrogen lines appear. 

 The experiments of Koscoe and Schuster, of Lockyer and other observers, with 

 potassium, sodium, and other metalloids in vacuum tubes, from which hydrogen is 

 pumped by a Sprengel pump, also show great changes in the molecular condition of 

 the mixture contained in the tubes when they are heated to different temperatures. 

 The changes of colour in the tube are accompanied by changes in the spectrum. Thus, 

 Mr. Lockyer finds that when potassium is placed in the bottom of the tube, and the 

 spark passes in the upper part of it, as the exhaustion proceeds and the tube is slightly 

 heated, the hydrogen lines disappear, and the red potassium line malces its appearance ; 

 then as the temperature is increased, the red line disappears, and three lines in the 

 yellowish-green make their appearance, accompanied by a change in the colour of the 

 tube, and at a higher temperature, and with a Leyden jar joined to a secondary 

 circuit of the induction coil, the gas in the tube becomes of a dull red colour, and 

 with this change a strong line comes out in the spectrum, more refrangible than the 

 usual red potassium line. In this case, on varying the conditions, we get a variation 

 in the character of the spectrum, and the colours and spectra are different in different 

 parts of the tube. In Lockyer's experiments, at the temperature of the arc obtained 

 from a Siemens djaiamo-machine, great differences appear in different parts of the 

 arc : for instance, Avith carbon poles in the presence of calcium, the band spectrum 

 of carbon, or the carbon flutings and the lines of calcium, some of them reversed, 

 are seen separated in the same way as mercury and hydrogen, the carbon spectrimi 

 appearing near one pole and the calcium near the other, the lines which are strongest 

 near that pole being reversed or absorbed by the quantity of calcium vapour sur- 

 rounding it. On introducing a metal into the arc, lines appear which are of different 

 intensities at different distances from the poles, others are strong at one pole and 

 entirely absent at or near the other, while solne lines appear as broad as half-spindles 

 in the middle of the arc, but are not present near the poles. Thus, the blue line of 

 calcium is visible alone at one pole, the 11 and K lines without the blue line at the 



other. 



AVe may probably regard these effects as the result, not of temperature alone, 

 but must take into account that we ha\e powerful electric currents which will act 

 unequally on the molecules of dift'erent bodies according as they are more or less 

 electro-positive. It would seem that we have here something analogous to the 

 segi-egation which is observed in the melting of certain alloys to which I have 

 already referred. 



The abundance of material in some parts of the arc surrounding the central 

 portion of it gives rise to reversal of the principal lines in varying thicknesses over 

 the arc and poles, so that bright lines appear without reversal in some regions, and 

 reversals or absorption lines without bright lines in others. The introduction of a 

 substance into the arc gives rise to a flame of great complexity with regard to colour 



