GRAPHITE. 



75 



from 2 to 3 jb. The latter is the one always observed and generally 

 attributed to oxides, in the case of a metallic arc. 



It is to be observed that the deflections are only a few millimeters as 

 compared with the large deflections when salts of metals are present. 



An examination was then made of the graphite arc, using a current of 

 4 amperes. The length of the image of the arc projected upon the spec- 

 trometer slit (which was also the length of the arc itself) was only from 

 2 to 3 mm. The arc was very unsteady and difficult to keep burning. 

 The results are given in fig. 52, curves a (4 amperes) and d. Curve a is 

 for an arc-length of 2 mm., while d is for a length of about 5 mm. For 

 the shorter arc the maximum is shifted toward the long wave-lengths. 



In this same figure, curve b gives the emission when the cold (negative) 

 electrode was a gas carbon 

 (with soft core) and the posi- 

 tive electrode was pure graph- 

 ite. The current was varied 

 from 5 to 6 amperes, and the 

 length of the arc was from 4 

 to 5 mm. There is an emis- 

 sion maximum at 4.6 /i, as 

 was found for the two graph- 

 ite electrodes. For compari- 

 son of position and intensity, 

 the emission band of carbon 

 dioxide in a Bunsen burner 

 is given in curve c, fig. 52. 

 The results with the pure 

 graphite show that there is a sharp emission band at 4.65 /* (4.55 fi for a 

 longer arc) , where previously, for a larger current, there was none. Since 

 all the observations extend over the same range of length of arc, it appears 

 that the occurrence of the emission band depends upon the current and that 

 it disappears for a large current. This is certainly extraordinary. Since 

 the observations on graphite were made in quick succession on the same 

 day, as given here, and since no changes were made in the apparatus, the 

 lack of an emission band for a current of 7 amperes is not to be attributed 

 to a fault in the adjustments. To attribute the occurrence of emission 

 bands, when salts of metals are present, to some "catalytic" action caused 

 by the presence of these metals does not elucidate matters very much. 



Paschen was the first to observe that the emission band of carbon 

 dioxide at 4.3 jx shifts to the long wave-lengths with rise in temperature. 

 At that time the dissociation of carbon dioxide at high temperatures was 

 but little investigated. Since the absorption band of carbon dioxide is at 

 4.3 n and that of carbon monoxide is at 4.6 /*, if the emission band is due 

 to a pure thermal effect, then one would expect to find that, with rise in 



SJU 4 SJU 



Fig. 52. Emission of graphite arc (4 to 6 amperes). 



