256 Mr. Gr. A. Hemsalech : Excitation of Spectra of Carbo?i, 



whicli are due the impurity lines observed under these con- 

 ditions, are located within the enclosed space — namely, in 

 the hot region extending down from the central part o£ the 

 plate. In order to account for the greater extension down- 

 wards of the line emission as compared with the continuous 

 emission due to the bluish vapour, we may suppose that the 

 emission centres, which are probably constituted o£ charged 

 particles (perhaps carbide molecules, see § 6), travel along 

 lines of force under the action of the electric field established 

 along the graphite plate by the passage of the heating current. 

 As it will be of advantage for the sake of clearness to desig- 

 nate the particular vapour which gives out the line emission, 

 as distinct from the bluish vapour, it will always be referred 

 to as the luminous vapour. 



With regard to the iron lines it will be shown hereafter 

 that their relative development is in accordance with that, 

 which I have previously observed for the same element in 

 flames and in the tube-furnace. From this we may conclude 

 that, also in the present case, their emission is caused by 

 thermo-chemical excitation — namely, the action of heat on a 

 chemical compound of iron, in this case probably a carbide. 



A characteristic feature of all the lines emitted by the 

 luminous vapour is that they die out gradually on passing 

 downwards. This is, of course, to be expected, if these 

 radiations are, as I presume, controlled by the plate tem- 

 perature, for the latter naturally decreases with increase of 

 distance from the plate. In this respect these lines behave 

 very similarly to what they do in flames, in the mantles of 

 which they are likewise observed to die out only gradually 

 on passing into cooler regions. 



When the plate temperature is raised to about 2700° C. 

 a new spectrum begins to develop in the immediate vicinity 

 of the graphite plate, with the appearance of the carbon* 

 bands at 3883 and 4216 and of numerous lines due to 

 titanium and vanadium. As the temperature is gradually 

 increased this spectrum gains in prominence and more 

 earbon bands appear in the visible part until nearly all the 

 bands of the Swan spectrum are out. Finally, in the red 

 part is seen a group of most intense, hazjr, and broad bands, 

 which when the spectrum is fully developed constitute its 

 most brilliant feature. Now the very striking and dis- 

 tinguishing character of this spectrum is, that the lines 



* Hunge and Grotrian have recently concluded that these bands are 

 due to nitrogen. As a result of my own experiments, "which were like- 

 wise made at atmospheric pressure and of which I hope to give an 

 account on a future occasion, I am unable to endorse their view. Nor 

 do my experiments indicate that the presence of N is always essential 

 for their emission. 



