453 On M. Goldstein's Observations on Spectra of Gases. 



discharge, but that the occurrence of sparks depends much more 

 on the pressure of the enclosed gas and on the length of the in- 

 terpolated spark-distance. As long as, in the space filled with 

 rarefied air, the discharge does not pass in a proper spark, only 

 the band-spectrum appears ; when the spark enters, the lines of 

 the line-spectrum also are seen. 



With respect to the formation of the spark and with it the 

 appearance of the line-spectrum under circumstances otherwise 

 the same, viz. equal pressure and equal spark- distance, I have as 

 yet not been able to verify in air any perceptible influence of the 

 dimensions of the tube in which the air is enclosed ; according 

 to M. Goldstein's observations, such an influence seems to be 

 present, just as I have observed it in hydrogen. In reference to 

 this, I take leave to quote a sentence or two from the forth-coming 

 new edition of the 2nd volume of my Experiment alp hysik, because 

 it at the same time explains the fact that, of two tubes simulta- 

 neously inserted in the circuit, with a capillary intervening por- 

 tion, the one tube containing air, and the other hydrogen, the 

 former shows the band-spectrum, the other the line-spectrum. 



After giving the explanation deduced by me from the obser- 

 vations on wide tubes, I say (p. 252) : — " The same difference 

 in the thickness of the luminous layer is also present in tubes 

 with an intervening capillary piece, as appears from the course 

 of the phenomena being the same in my experiments on the ni- 

 trogen-spectrum (Pogg. Ann. vol. cxxxvii.) ; there also the thick- 

 ness of the layer of gas which fills the capillary tube is still very 

 great in comparison with the fine line of the proper spark. 

 That some gases, rendered incandescent by the induction-current, 

 give only the line-spectrum depends on this, that the current 

 can only pass through them in sparks. It is worthy of note that 

 in narrow tubes, even with hydrogen, in slight pressures the 

 spark-discharge appears together with the sparkless one; in a tube 

 of 1 centim. diameter I almost constantly saw the spark- 

 discharge ; and in tubes with a capillary between them, in slight 

 pressures the spark-discharge often came alone, without that 

 luminous tuft. This circumstance accounts for the fact that, in 

 Geissler tubes with a capillary between-piece, we often obtain 

 only the line-spectrum, and often the same accompanied by the 

 band-spectrum." 



Hereby, probably, the most important of M. Goldstein's ob- 

 jections are answered. With respect to the thick sparks giving 

 the line-spectrum (p. 340), I only add that, leaving out of view 

 the fact that it is very difficult to give a definite judgment on 

 the thickness of a spark, every spark (if we designate as such an 

 entire discharge apparently taking place in one spark) consists 

 of a great number of partial discharges following one another in 



