SPECTEA OF IGNITED GASES AND VAPOUES. 
21 
of nitrogen (originating from the decomposed gas), whether we supplied the flame by a 
jet of oxygen, or operated in open air; but in both cases there is no reason not to 
admit that the bands, which are not seen in the case of any other compound of carbon, 
were owing to the undecomposed cyanogen (see no. 61). 
57. With regard to the spectrum of hydrogen , we first refer to former observations. 
The spectrum one of us obtained by sending the discharge of Ruhmkorff’s small 
induction coil through one of his highly evacuated spectral tubes, constructed by 
M. Geissler, shows only three bright lines, which he denoted by Ha, H/3, and Hy. 
The beautiful red light of the ignited rarefied gas, divided into these three bright lines, 
even after having passed through the four prisms of Steinheil’s spectral apparatus, 
remains highly concentrated. At a magnifying power of 72, the three bright lines or 
small bands thus obtained are well defined. Their apparent breadth is equal to the 
breadth of the slit ; consequently, on further narrowing the slit, they approach gra- 
dually to mathematical lines. Hence we conclude that, under the above-mentioned 
conditions, the length of wave of the light constituting each of the three hydrogen-lines 
is constant, and remains so if by widening the slit the lines are expanded into bands. 
In referring the middle lines of such bands to the middle line of the direct image of 
the slit, we obtain its angle of refraction. It was proposed to employ these middle 
lines instead of Fraunhofer’s dark lines of the solar spectrum in determining the indices 
of refraction*. This proceeding has since been proved to be very expedient f. 
58. Hydrogen permits the electric discharge to pass at a lower tension than other 
gases do. When Ruhmkorff’s small induction coil was discharged through a spectral 
tube enclosing hydrogen, which was gradually rarefied to the highest tenuity to be 
reached by means of Geissler’s exhauster, finally the beautiful red colour of the 
ignited gas became fainter, and passed gradually into an undetermined violet. When 
analyzed by the prism, Ha disappeared, while H/3, though fainter, remained well defined. 
Accordingly light of a greater length of wave was the first extinguished $. 
59. Hydrogen shows in the most striking way the expansion of its spectral lines, and 
their gradual transformation into a continuous spectrum. When the direct discharge 
of Ruhmkorff’s large induction coil is sent even through the old spectrum tubes 
enclosing hydrogen, the formerly obtained spectrum is essentially altered. By increas- 
ing the power of the coil, the violet line Hy first expands ; while it continues to 
expand, the expansion of the bluish-green line H/3 becomes visible. Let the aperture 
of the slit be regulated so that the double sodium-line will separate into two single 
lines nearly touching one another. Then, the angular breadth of H/3 becoming two or 
three minutes, the breadth of Hy is about double. The expansion takes place as well 
* Poggendorff’s * Annalen,’ vol. cvii. p. 497. 
t Landolt: “Ueber die Breehungsexponenten flUssiger homologer Verbindungen,” Poggendorff’s ‘Annalen/ 
vol. cxvii. p. 353. 
t Plucker : “ Ueber recurrente Strome und ihre Anwendung zur Darstellung von Gasspectren,” Poggen- 
dorff’s * Annalen/ vol. cxvi. p. 51. 
