1872.] Mr. A. Schuster on the Spectrum of Nitrogen. 487 



repeated several times in succession, but invariably with the same result. I 

 have in my possession two tubes sealed off under 2 millims. pressure, one 

 without sodium, showing the fluted bands, the other containing sodium, 

 showing the spectrum of lines. Two other tubes, sealed off under 1 5 millims. 

 pressure, show the same thing. I have repeatedly convinced myself that, 

 from the highest pressure under which the spark of the induction-coil passes 

 to the lowest pressure which I could obtain with an ordinary air-pump, pure 

 nitrogen invariably gave one and the same line-spectrum. Once, when I 

 intended to seal a tube off under higher pressures, it was found that the 

 sodium was not sufficient to absorb all the oxygen present, so that a sort 

 of mixture of the two spectra was seen. Such a mixture was often observed 

 by Pliicker and Wiillner at the point where one spectrum changed into 

 the other ; it is characterized by the green line of nitrogen and the fluted 

 violet bands at the same time. 



The tube showing the mixture at 15 millims. pressure was gradually ex- 

 hausted, but the spectrum remained exactly the same. If the formation 

 of the two spectra depends merely upon the pressure or temperature to 

 which the gas is subjected, how can a mixture of the two spectra, indicating 

 a state of transition, exist under so entirely different pressures and different 

 temperature? 



In order to ascertain whether nitrogen even carefully prepared contains 

 oxygen, a drop of a solution of iodide of potassium and starch was intro- 

 duced into the tube ; after the spark had passed for a few seconds only, 

 the liquid was coloured blue — showing either the formation of oxides of 

 nitrogen or of ozone, but at any rate the presence of oxygen. 



7. Spectrum of oxides of nitrogen. — I tried to obtain the spectra of the 

 different oxides of nitrogen ; they all give the same fluted spectrum, and I 

 could get no information as to which particular oxide the fluted spectrum 

 is due : this is, however, easily understood if we remember that it is just as 

 difficult to prepare the oxides of nitrogen free from oxygen as pure nitrogen 

 itself ; so that the oxide giving the spectrum in question will always be 

 formed. I have, however, convinced myself tbat the absorption-bands of 

 nitrous acid gas are not coincident with the bright bands of the spectrum ; 

 and it is probable that the spectrum is due to nitric oxide, this being the 

 most stable of all the oxides of nitrogen. 



I may add that one of the tubes containing the sodium and showing the 

 lines one day cracked, and then at once showed the violet bands. This 

 fact will not be easily explained by the assumption that the fluted spectrum 

 belongs to a lower pressure and lower temperature than the spectrum of 

 lines. 



I propose to subject the different spectra of the remaining gases to a 

 careful examination. 



The above experiments were made in the Physical Laboratory of Owens 

 College, Manchester ; and I have to thank Professors Balfour Stewart and 

 Roscoe for many valuable suggestions. 



