on Spectra of Gases. 449 



rhythm of the discharge was present in every part of the cir- 

 cuit, M. Goldstein believes that when at one place there is a 

 spark it also appears at all places in the circuit at which inter- 

 ruption takes place, that consequently, according to my explana- 

 tion of spectra, the line-spectrum would everywhere show itself. 



I see, on the contrary, in the experiments of M. Goldstein in 

 general a corroboration of my view, which requires a band- 

 spectrum whenever extensive masses of gas are rendered lumi- 

 nous. The error in M. Goldstein's assumptions is this : — When 

 at one place in the circuit the discharge passes in a spark, he 

 infers that it must also do so in all spectral tubes inserted, 

 because the rhythm of the discharge is everywhere the same. 

 This is not the case; the form in which the discharge takes place 

 in spectral tubes depends on the pressure of the gas and on the 

 dimensions of the tube. 



That like rhythm does not establish like form, I have already 

 shown in my treatise on the origin of spectra of different orders 

 (Pogg. Ann. vol. cxlvii. p. 337), where, in one and the same 

 hydrogen-tube, the spark extended only from the positive elec- 

 trode about halfway down the tube, but below this it was dis- 

 solved. If the slit of the spectrometer was on a level with the 

 spark, it gave the line-spectrum ; if it was in front of that part 

 of the tube in which the spark was lost, the band-spectrum 

 was seen. 



M. Goldstein himself has also observed that in spaces con- 

 taining air sufficiently rarefied, in spite of the insertion of a 

 spark-distance, no spark comes, when he (in p. 603 of his com- 

 munication) speaks of several centimetres thickness. He even 

 alternated sparks with a discharge as rapid as the spark. That 

 (as M. Goldstein truly remarks) such a discharge does not give 

 a line-spectrum, is the best proof of the correctness of my inter- 

 pretation of the phenomena ; for in that case the whole of the 

 light filling the tube shines, and not, as in proper sparks, merely 

 a few molecules lying in the line of the spark. 



Last year I made a great number of experiments on the 

 passage of the induction-current through tubes filled with 

 rarefied gases; and in those experiments, exactly as M. Goldstein 

 did, I interposed spark- distances, and sometimes also Leyden 

 jars. I have not been able to finish those experiments, because 

 working at the new edition of my E experiment alp hy si k has 

 claimed the whole of my attention. I have on this account 

 communicated only a small portion of them, treating of the 

 forms of the positive luminous tuft, in tubes filled with air, in 

 its dependence on pressure and on the dimensions of the tube. 

 Let me be permitted here to communicate a series of experiments 

 from March 1873, carried out in conjunction with Dr. Winkel- 



Phil. Mag. S. 4. Vol. 49. No. 327. June 1875. 2 I 



