368 M. A. Wiillner on the Spectra of some 



the continuous spectrum just described really belongs to hydro- 

 gen, and not to incandescent solid particles carried by the current. 



b. Oxygen. 



At low pressures M. Wiillner obtained with a large Ruhm- 

 korfFs coil and his six Grove's elements results quite agreeing 

 with those previously obtained. At 10 millims. he observed a 

 spectrum of the second class identical with that which Plucker had 

 described; but beyond that, and in proportion as he increased 

 the pressure, this spectrum lost more and more of its lustre. At 

 200 millims. scarcely more than a small portion of the lines com- 

 posing it were perceived, those contained in the green and the blue. 

 From this point the luminous intensity again increases, Plucker' s 

 spectrum-lines gradually reappear, and at the same time there is 

 seen behind them a continuous spectrum, which is first seen in 

 the green and then gradually spreads out, especially on the side 

 of the more refrangible rays. At 650 millims. the continuous 

 spectrum extends from the ray called Oa by Plucker to the 

 middle of the interval between Fraunhofer's lines G and H. It 

 is particularly bright in the green part, where its lustre almost 

 causes the bright lines of the spectrum of the second class to 

 disappear. Yet these, contrary to what was observed in the case 

 of hydrogen, do not lose the definiteness of their contour, nor 

 become broader, but are less well distinguished on a ground which 

 is almost as brilliant as they. With RuhmkorfFs inductorium 

 M. Wiillner could never with oxygen exceed a pressure of 800 

 millims. The continuous spectrum remained almost the same, 

 with a rather more equal lustre in its different parts; and the 

 bright lines always stood out with perfect precision on this lu- 

 minous ground. Here also, though the lustre increased, the 

 spectrum never extended beyond the limits within which it was 

 contained at low pressures. 



The use of a Leyden jar gave in this case the same results as 

 the simple induced current, but with a far higher degree of inten- 

 sity. At 30 millims., with a Leyden jar we have a very bright 

 continuous spectrum quite resembling that which the induced 

 current gave at 600 millims. As the pressure increases, the 

 lustre of the continuous spectrum and of the bright lines increase 

 at the same rate, especially in the orange and yellow parts. 

 There, as in the case of hydrogen, the bright lines lose the pre- 

 cision of their contour and seem to spread out. At 280 millims. 

 this part of the spectrum forms a continuous field, while every- 

 where else the bright lines continue, and stand out on a basis a 

 little less bright than they. The phenomenon remains constant up 

 to a pressure of 540 millims., which M. Wiillner never exceeded 



