298 M. F. Zollner on the Temperature and 



there reappeared a portion of the continuous spectrum in the 

 green, in the form of perhaps five bright fields, when the gas 

 was rarified to fractions of a millimetre pressure"*. 



In a subsequent memoir, " On the Spectra of some Gases 

 under high Pressures "t> Wullner observed the spectrum of 

 hydrogen up to a pressure of 2240 millims. > the highest which 

 his apparatus permitted. The most essential characters of 

 the spectrum with 1703 millims. pressure are described as fol- 

 lows : — 



"With this pressure H« has already lost much of its sharp- 

 ness ; it shows itself as a band of several minutes breadth, at the 

 edges of which the intensity of the light diminishes quickly. . . . 



" The green portion of the spectrum shines very bright ; the 

 brightness increases at first more slowly, then more quickly as 

 far as the place of Up, where the spectrum is brightest, so that 

 this part appears almost white. In the direction of the blue the 

 brightness lessens quickly; yet the blue and violet are very 

 beautiful, most so in the region of H y , so that, compared with 

 the appearance with a lower pressure, H y seems to reappear, 

 though without sharpness and widened like Hp. On this ac- 

 count the boundary of the spectrum goes a little beyond the 

 place of H y ." 



Passing then to the highest pressure, he remarks as follows : — 



"By a still further augmentation of the pressure a much 

 nearer approach to a continuous spectrum could not be attained ; 

 even with' a pressure of 2240 millims., or almost 3 atmospheres 

 (the highest permitted by the dimensions of the apparatus), H a 

 still persisted in a similar fashion ; the loss of sharpness at the 

 edges, however, had proceeded so far that we may expect with a 

 still further increase of pressure to see H a disappear, just as 

 Hp and H y have alreadv vanished under less pressures " (/. c. 

 p. 342). 



In his most recent paper " On the Spectra of Gases in Geiss- 

 ler's Tubes " (Pogg. Ann. vol. cxlvii. p. 347), M. Wullner seems 

 inclined to assume that the change described by him in the above 

 extracts, of a discontinuous into a continuous spectrum is "essen- 

 tially dependent on the rise of the temperature." Among the 

 reasons which compel him to such an assumption, he alleges the 

 following :- — 



"That it is in reality the rise of the temperature which deter- 

 mines the formation of the continuous spectrum is also spoken 

 for by the fact that in most cases it is accomplished not by a 

 widening of the bright lines, but by the illumination of the entire 

 background constantly growing brighter, without the lines mean- 



* Pogg. Ann. vol. cxxxv. pp. 305 & 306. 



t P°Kg- Ann. vol. cxxxvii. p. 337. F*liil= Mag, S. 4, vol. xxxix. p. 365, 



