the Occurrence of Enhanced Lines in the Arc. 303 



of the spectra of lead, a, b, and c were obtained with a 

 carbon cathode (top) and Pb anode, a is the spectrum of 

 the ordinary arc in air, showing the usual development 

 under these conditions. The narrow strip of continuous 

 spectrum along the upper edge is caused by the white-hot 

 tip of the carbon cathode. There are no traces of spark 

 lines in this spectrum, b was obtained with the same elec- 

 trodes as a, but a current of air was blown through the arc 

 from the moment of striking the same. The strip of con- 

 tinuous spectrum seen near the anode is produced during 

 the first phase of the discharge ; it is also at this stage that 

 the spark lines X4245 and X4387 are most strongly de- 

 veloped. Arc as well as spark lines appear symmetrically 

 widened or winged in the first phase. During the second 

 phase the spark lines are relatively feeble, but they remain 

 visible till the end of the arcing ; air is of course passing- 

 through the arc all the time and keeps its ionization at a low 

 value. Spectrum c was obtained with a drop of distilled 

 water placed between a carbon cathode (top) and a Pb anode. 

 In the first phase the spark lines X 4245 and X 4387 are 

 relatively stronger than the arc lines X4020 and X4168. 

 During the second phase the spark lines die out sooner than 

 either flame or arc lines. Noteworthy is al^o the relativelv 

 great reduction in intensity of the flame line X 4058. The 

 band at X4216 shows faintly throughout the second phase 

 on the original negative. Spectra d and e were obtained 

 with Pb electrodes in oxygen and coal-gas respectively. In 

 both spectra the spark lines are strongest at the beginning 

 of the arc flash when the poles were still near together; but 

 thev are likewise, though less, intensely, emitted throughout 

 the second phase. In coal-gas they are particularly strong 

 and are only surpassed in intensity by the flame line 

 A 4058. 



Plate V. gives the red region of the arc spectra of tin as 

 obtained under various conditions. Spectrum a is that of an 

 ordinary arc between a carbon cathode (top) and an Sn 

 anode ; no trace of a spark line is to be seen, b is the spec- 

 trum of an ordiaary arc between a graphite anode (top) and 

 an Sn cathode : a number of spark lines are revealed in the 

 vicinity of the metal cathode. These lines are relatively 

 strengthened in the blown graphite arc as shown in c, With 

 the water arc the spectrum consists predominantly of spark- 

 lines (spectrum d). A feature of this spectrum is the great 

 intensity of the hydrogen line X 6563. 



