Gases under High Pressures. 367 



three atmospheres) it almost exactly resembles a solar spectrum 

 restricted to Fraunhofer's lines C and G*. It merely differs by 

 the presence of the line Ha, which, however, is much widened, 

 and by the maxima of luminous intensity which it presents in 

 the places where were the lines H/3 and Hy. 



M. Wiillner concludes, from what has been said, that above a 

 certain density the temperature of the jet, and with it the lumi- 

 nous intensity of the spectrum, increases continuously ; so that 

 we are warranted in assuming that, by subjecting the gas to a 

 still higher pressure, or by raising the temperature of the jet, we 

 should obtain a continuous spectrum without any bright line. 

 He endeavoured to effect this by introducing a Leyden jar into 

 the induced circuit, so that it was discharged between the two 

 electrodes nearest to one another. With the Leyden jar, and in 

 proportion as the pressure is increased, the spectrum of hy- 

 drogen is seen to pass through almost the same phases as when 

 a powerful E-uhmkorff's inductorium is used; but the successive 

 transformations of the spectrum are more rapidly effected. Thus 

 at 300 millims. with the Leyden jar, the same appearance is pre- 

 sented as under three atmospheres with the induction-coil alone. 

 At 560 millims. the spectrum is almost continuous. Ha is no 

 longer a bright line, but a broad red band ; and H/3 is no longer 

 recognized, as before, by a maximum of luminous intensity. At 

 1000 millims. the spectrum becomes still more equalized; there 

 is at most a maximum lustre where formerly the line H a appeared; 

 the temperature of the jet is so raised that the sodium-line appears 

 distinctly as a bright line. Lastly, at 1230 millims. the lustre 

 of the jet is so great that the sodium-line is reversed, and ap- 

 pears just like Fraunhofer's line D in the solar spectrum. Thus 

 it is seen that an incandescent gas may produce a reversed 

 spectrum, provided its temperature is high enough, and that it is 

 not necessary for the principal source of light to be an incan- 

 descent solid. 



The author pushed the experiment as far as a pressure of 

 1320 millims., at which the discharge of the Leyden jar became 

 discontinuous. The appearance of the spectrum at this pressure 

 was still the same, as was also its length. From the commence- 

 ment of the experiment, at low pressures a spectrum was ob- 

 tained restricted to the lines H a and H 7 ; and its limits remained 

 constant during the whole of the experiment. It is not likely 

 that the spectrum of hydrogen would ever extend further^ even 

 at the highest pressures ; we have here even a good criterion that 



* With this limitation, however — that the spectrum of hydrogen at high 

 pressures is a direct spectrum with no dark line, while the solar spectrum is 

 reversed. 



