xn.j COINCIDENT LINES. 153 



not by reducing the pressure, but by reducing the quantity of any 

 particular gas in a mixture. 



If we take, for instance, a spark in air and observe its spec- 

 trum, we find the lines of the constituents of atmospheric air 

 considerably thick ; but if we wish to observe accurately the 

 lines of one of the constituents, say oxygen, these should be fine, 

 in order to enable us to determine their absolute position. To 

 accomplish this, the spark is taken in a glass vessel with two 

 adits and one exit tube. If we wish to observe the oxygen lines 

 fine, the vessel is flooded with nitrogen so that there is only a 

 small quantity of oxygen present, and pass the current between 

 the inclosed electrodes. If we wish to observe nitrogen lines 

 fine, it" is flooded with oxygen, so that there is only a small 

 quantity of nitrogen present. 



In this way, by merely making a mixture in which the 

 gas to be observed is quantitatively reduced, so that the lines 

 which we have to investigate are just visible in their thinnest 

 state, we have a perfect means of doing it without any appa- 

 ratus depending on the use of low pressures. A very great 

 simplicity of work is thus introduced. 



2. The Long Lines are in some cases common to many Spectra. 



While on the one hand, as we have seen, the short lines are 

 reduced in number with reduced quantity of the substance 

 producing them, it soon became apparent on the other, that the 

 long lines had a trick of making their appearance in many 

 spectra. Any one who will consult Thalen's tables, or Kirch- 

 hoff's or Angstrom's map, will note the many coincidences. 

 This point, then, deserves careful study. 



The following general statements were soon hazarded, with 

 the proviso that it was possible that further inquiry might modify 

 them. 



1st. If the coincident lines of the metals are considered, 

 those cases are rare in which the lines are of the first order of 



