ABSORPTION 1 AND EMISSION CENTERS. 5 



possible, then, by its frequent collisions with free electrons. On the other 

 hand, band spectra, which are not in general affected by electric fields, Stark 

 considers as being due to the combining of a positive ion and a negative 

 electron. During this recombination there is a considerable decrease in the 

 potential energy of the system, and it is from this energy that the band spec- 

 trum is produced. 



In their early work Stark and Riecke 1 describe an arc-like discharge 

 between copper electrodes of about 3600 volts difference in potential. The 

 vapors of sodium or lithium salts placed in this discharge spread towards 

 the negative wire, thus indicating that the carriers are positive; this being 

 a direct contradiction of Lenard's results. 



Most of Stark's later work has been done with canal rays. They move 

 towards the cathode and pass through any openings in it, with a velocity of 

 about 10 s cm. per second. If there is hydrogen in the tube both the line and 

 band spectra of hydrogen may be seen. In nitrogen the line spectra are diffi- 

 cult to obtain. The lines of hydrogen, nitrogen, mercury, sodium, potassium, 

 etc., are found to show the Doppler effect when viewed in the direction in 

 which the canal-ray particles are moving. 



The substances showing the Doppler effect also give spectra that do not 

 show the Doppler effect, i.e., there are rest lines and shifted lines. The rest 

 line is usually much the sharper of the two, and the space between the two 

 lines is usually dark. The line shifted towards the violet usually has its sharp- 

 est side on the red. The rest line comes largely from the gas through which 

 the canal rays are passing. The displaced line is composed of light radiated 

 by the canal-ray particles themselves, and since these lines are hazy, the canal 

 rays are moving with different velocities, according to the part of the "cathode 

 fall" region from which the particles originate. The particles that traverse 

 the whole region of the cathode fall will accordingly have the maximum veloc- 

 ity. Having passed through a field of from 300 to 500 volts, the velocity of 

 the canal-ray particles becomes sufficient to produce ionization by collision. 



Knowing the cathode fall, the mass of the canal particle, and the maxi- 

 mum displacement, Stark calculates the charge of the canal particle. For the 

 hydrogen series, the principal and subordinate series of sodium and potas- 

 sium, and for certain lines of mercury, the value of the charge, according to 

 the earlier papers of Stark, is the same as that of the elementary charge of 

 electricity. Stark also considered the mercury triplets to be due to particles 

 carrying a double charge, and the mercury line X 4078.1 to be due to a carrier 

 having more than two charges. 



THE CARRIERS OF SPARK SPECTRA. 



Among investigators who have taken up the problem as to what is the 

 nature and velocity of the particles in sparks that are acting as absorption and 

 emission centers of line spectra, may be included Schuster and Hemsalech," 

 Schenck, 3 Royds, 4 Milner, 5 Miss Schaeffer, 6 and others. These workers have 



1 Phys. Zeit., 5, 537 (1904). 



2 Phil. Trans., 193, 209 (1900); Compt. Rend., 142, 1511 (1906). 



3 Astrophys. Journ., 14, 110 (1901). 



4 Phil. Trans., 208. A. 333 (1908). 

 5 Ibid., 209, 71 (1908). 

 "Astrophys. Journ., 28, 121 (1908). 



