on Spectra of Gases. 451 



shortening the spark-distance the clouds gradually develop ; and 

 when it is reduced to 20 millims. they are already completely 

 formed, but are of shorter duration than when there is no spark- 

 distance. With 65 millims. pressure the number of clouds rises 

 to five or six; in other respects the appearance is substantially 

 as before. 



Pressure 100 millims. Without a spark- distance, essentially 

 a positive luminous tuft, in the shape of a great number of clouds, 

 which give the band -spectrum. When a spark-distance of 10 

 millims. is interpolated, there appears a small and feeble spark, 

 in the rotating mirror as a fine line of white light opening the 

 discharge; but it reaches only about 1 centim. below the positive 

 electrode, and is there lost in the diffused discharge, as before de- 

 scribed in the case of hydrogen. On increasing the spark-dis- 

 tance inserted, the spark extends further; with one of 30 millims. 

 it sometimes goes quite over from one electrode to the other. 

 When the spark enters, the green lines of the line-spectrum im- 

 mediately appear in the band-spectrum otherwise furnished by the 

 tube, and exactly as far as the spark extends ; so that in the spec- 

 trum the length of the spark can be accurately ascertained, even 

 without the aid of the rotating mirror. 



Pressure 145 millims. Without the interpolation of a spark- 

 distance the current passes at first in the form of a partial dis- 

 charge; then follows a positive tuft which appears in the rotating 

 mirror as flickering clouds. The insertion of a spark-distance 

 of 12 millims. sometimes produces in the tube a very peculiar 

 spark-discharge : from the positive electrode springs a spark to 

 about the distance of 1 centim. in the tube, and is there lost, 

 but reappears 1 centim. lower down, looking again like a spark- 

 line 1 centim. in length. Sometimes the spark is present as a 

 line along this whole distance ; but it never goes further than 

 about half the distance between the two electrodes. Only with 

 the external interpolation of a greater spark-distance does the 

 spark in the tube become longer ; and when the former amounts 

 to 30 millims. the latter sometimes extends the whole distance 

 from one electrode to the other. The spectrum of the spark 

 shows, besides the green, even the yellow and red lines of the 

 nitrogen line-spectrum. The lines, however, are to be seen in 

 the spectrum only just as far as the spark extends (here, as before, 

 I copy, word for word, the note made at the time) ; hence we 

 can recognize in the spectrum precisely to what distance the spark 

 reaches. 



The above-related series of experiments consequently shows 

 unequivocally that a spark-distance interpolated in the circuit of 

 the induction-current by no means always calls forth sparks in 

 spaces filled with rarefied gases, even with uniform rhythm of the 



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