144 THE ELECTRIC ARC 



cathode. Hull 1 makes a similar statement for currents of 

 only a few amperes, but states that with the hissing arc 

 the metallic spectra appeared at both poles. 



When metals were introduced into the arc, Miss Baldwin 

 found that the more positive ones, such as potassium, 

 sodium, lithium, barium, strontium and calcium appeared 

 at the cathode more brightly than those less electropositive, 

 such as cadmium, zinc, copper and silver. 



One of the most important observations on the spectrum 

 of the arc, provided it is correct, was made by Lenard. 2 

 He states that the arc consists of different layers and that 

 different series of lines of the spectrum of a given element 

 appear in these different layers. Thus, if sodium is intro- 

 duced into the arc, only the principal series of the spectrum 

 appears in the outside layer. The first subordinate series 

 of lines appears in an inner layer of vapor and the second 

 subordinate series in a layer within this. An absorbing 

 medium which cuts out all of the lines except the principal 

 series, shows all of the vapor of the arc. A medium leaving 

 only the first subordinate series will show an inner cone, 

 but not the whole luminous vapor. 



Lenard's explanation is that the atoms of an element are 

 acted on differently in different parts of the arc and that 

 each series is produced by one particular method of ex- 

 citation. The experiment itself has been verified by 

 Puccianti. 3 On the other hand Kayser 4 does not agree 

 with Lenard but thinks that what he observed was due to 

 varying intensity of the different lines in different parts of 

 the arc. 



1 Astro-phys. Journ., 28, 59; 1908. 



2 Ann. d. Phys., n, 636; 1903. 



* N. Cimento, (5), 13, 268; 1907. Phys. ZS., 8, 463; 1907. , 

 4 Sz. wiss. Phot., 6, 68; 1908. 



