Second Spectrum of Hydrogen. 333 



the second spectrum appeared at pressures of 22 millim. and 

 2 to 3 metres respectively. 



These views of the connexion between the process of dis- 

 charge and the nature of the spectral phenomena obtained 

 have already been propounded by Angstrom*, being supported 

 by observations of the image of the tube produced by a rota- 

 ting mirror. But if we observe in this manner, not the tube 

 itself as is generally done, but the spectrum of the tube, it 

 would seem that we ought to obtain a still sharper criterion of 

 the admissibility of the above view than is afforded by experi- 

 ments hitherto described. If the three principal lines are to 

 be referred exclusively to the momentary chief discharge, and 

 the system of lines forming the second spectrum to the series 

 of partial discharges, then, in the revolving mirror, the first 

 should appear as isolated lines upon a continuous background 

 formed by the latter. However simple the experiment theo- 

 retically considered may appear, its actual performance is 

 attended by such considerable difficulties, in consequence of 

 the small luminous intensity of the spectrum, that my attempts 

 in this direction have not given any certain result. 



Thicker and Hittorf, from their experiments with wide 

 tubes, drew the conclusion, in respect to the origin of the 

 second spectrum, that it is to be ascribed to hydrogen just as 

 much as the four lines undoubtedly characteristic of this gas. 

 In their method of experimenting there is, in fact, no un- 

 certainty upon this point arising from possible influence of 

 the material of the electrodes or of the glass envelope. Al- 

 though no details are given of the method of preparing the 

 gas, yet it is clear that Pllicker and Hittorf have certainly 

 bestowed due care upon this point, so that their conclusions 

 need not fear criticism upon this ground. Since these con- 

 clusions were very completely confirmed some years later 

 by the experiments of Willi ner |, there was every reason to 

 regard this question as settled. But other views were ex- 

 pressed almost at the same time, based upon chemical inves- 

 tigations on the conditions of equilibrium of gaseous systems, 

 by Berthelol and Richard J, which not only caused the re- 

 opening of the question, but so hindered a definite decision 

 that even now there is no agreement among spectroscopists 

 upon this point. I will now consider these views more 

 closely. 



It is known that every compound body is decomposed into 



• a R t. lxxiii. (1871), p. 871. 



t Pogg. Ann, vol. cww. p. l!>7, cxxxvii.p. 337. Festschrift der Nieder- 

 rhein, (res. f. Natur- und Heilkunde, Bonn, March 1858. 

 \ C. B, Ixviii. pp. 810, 1035, 1107, 1510. 



