SPECTRA OE IGNITED GASES AND VAPOURS. 
5 
electricity depends essentially upon their temperature. At the ordinary temperature it 
is rather too great to be measured, but, according to hitherto unknown laws, it rapidly 
decreases when the temperature rises beyond that of red heat. The law above men- 
tioned is therefore not strictly applicable in the case of gaseous conduction. 
11. Electricity can only be discharged through a given stratum of air, from one point 
to another, after a certain electric tension takes place in these points. This tension 
depends upon the chemical constitution of the gas, and, the gas being the same, it is 
nearly in the ratio of its density and the distance of the two points. The quantity of 
electricity required to produce that degree of tension which must precede the electric 
discharge through our spectral tubes, enclosing gas of a given density, may be inde- 
finitely increased by interposing a Leyden jar. The less the distance between the coat- 
ings of the jar, and the larger their surface, the greater quantities of electricity will be 
accumulated on them, ready for discharge at the moment when the electric tension of 
the electrodes entering our tube reaches that intensity which alone allows the discharge 
to take place. Thus the Leyden jar is the most proper and most easy means for short- 
ening the duration of the discharge, and consequently increasing the temperature of 
the gas. 
In several cases, especially if a vapour like that of mercury be examined, which 
isolates less, it will be found more convenient, instead of replacing the Leyden jar by a 
larger one, to increase the charge of the same jar by intercalating into the circuit a spark 
micrometer, by means of which you may add to the resistance within the spectral tube 
the resistance of any stratum of air. 
12. The leading idea by which one of us was guided when he first (1857) directed 
his attention to spectral analysis, was to concentrate the light in Geissler’s tubes by con- 
fining the electric current within a capillary channel *. The construction of our tubes 
immediately follows from it. Accordingly we gave, for different purposes, a different 
diameter to their capillary part. The length of this part is of very little influence if 
the tubes are very highly exhausted ; we had to shorten our recent tubes, intended to 
enclose gases and vapours of a greater density, rendered luminous by a powerful induc- 
tion coil. 
13. We employed in our researches the large spectral apparatus constructed by 
M. Steiniieil. The refracting angle of one of the four flint prisms belonging to the 
apparatus is 60°, the angle of the three others 45°. Generally we made use of only two 
prisms (of 60° and 45°), and of a magnifying power of only 18. 
It is well known that the slit of the apparatus, if illuminated by sodium-light (by the 
flame of alcohol containing common salt), is seen double. According to the width of 
the slit and the dispersive power of the prisms, the two well-defined images, having both 
* Plucker : “ Spectra der elektrischen Licbtstromungen,” 30 Marz 1858, Poggendorff’s ‘ Annalen,’ vol. civ.; 
“ Ueber die Spectra der verschiedenen Gase, wenn durch dieselben bei starker Verdiinnung die elektriscbe Ent- 
ladung bindurchgebt,” 25 Aug. 1858, Ibid. vol. cv.; “ Ueber die Constitution der elektriscben Spectra von ver- 
scbiedenen Gasen und Dampfen,” 5 Mai 1859, Ibid. vol. cvii. 
