SPECTRA OF IGNITED GASES AND VAPOURS. 
3 
cultly fusible, and bent as shown by the diagram (fig. 2). After having introduced into it 
a small quantity of the substance, the last traces of air were expelled from the tube, which 
was finally blown off. Put before the slit of the spectroscope, the enclosed substance 
was, by means of a lamp, reduced into vapour and, if necessary, kept in the gaseous 
state (fig. 3), and the density of the vapour regulated. The glass of our spectral tubes 
of this description is fused with such difficulty, that these highly evacuated tubes, when 
becoming red-hot by the lamp, are not altered by the pressure of the surrounding air. 
Fig. 2. Fig. 3. 
7. Before giving a general account of the results we have obtained, it seems necessary 
to enter into some preliminary discussions regarding the admirable working of Geissler’s 
exhauster, and the phenomena shown by our tubes when highly evacuated by it. The 
essential part of Geissler’s exhauster is a large glass ball, containing ten to twenty 
kilogrammes of mercury, which in its upper part communicates, by means of a doubly 
perforated stopcock of glass, either with the free air, or with the spectral tube to be 
evacuated. From the lower part of the ball, which is invariably fixed, descends a longer 
tube of glass communicating at its lower extremity with a moveable similar tube, the 
free end of which enters into a large open bottle. When this bottle with the moveable 
tube is lifted up, the mercury within the apparatus entirely fills the ball, if commu- 
nicating with the air. This communication having been interrupted, a Torricellian 
vacuum is formed when the bottle descends. By establishing the communication with 
the spectral tube, the gas within it will be dilated. After the ascent and descent of mer- 
cury has thus been alternately produced often enough, no perceptible trace of air will 
remain within the spectral tube. 
8. A tube evacuated in this way does not permit the induction current of Ruhmkorff’s 
smaller apparatus (which in air gives a spark of about 15 millims.) to pass through. 
The current of his large apparatus forces a passage ; but the spectrum we obtain in this 
case is very faint ; it shows no traces of the bands of nitrogen, but solely the lines of 
hydrogen and the large fields of vaporized carbon (51). The hydrogen-lines take their 
origin from hygroscopic water covering the interior surface of the spectral tube, the 
carbon-bands probably from the minute traces of fatty matter hitherto employed in 
b 2 
