LIGHT AND HEAT 245 



vibrations have only a very small amplitude. Koenigsbcrger and 

 Kiipfur and others consider that the band spectra of iodine, bromine, 

 nitrogen peroxide (NoO^), sulphur, iodine trichloride, nitrogen, etc., 

 are due to a dissociation or recombination of the respective molecules, 

 atoms or ions. In the case of iodine this change might be represented 

 by the equation 



I^^I + I. 



At about 800° C. this reaction is about complete and the fme-banded 

 absorption spectra should therefore disappear. Galitzin, Wilip, Evans 

 and others have shown that the bromine absorption spectrum disappears 

 as dissociation becomes more and more complete. 



Caxal Eays 



Canal rays have their source in positive ions that start in front of 

 the cathode, move towards the cathode and pass through any openings 

 in it with a velocity of about (10)^ cms. per sec. After passing the 

 cathode the canal ray particles may lose their charge or even become 

 negatively charged. The spectrum lines of hydrogen, nitrogen, mer- 

 cury, sodium, potassium, etc., emitted by canal rays show the Doppler 

 effect when they are viewed in the direction in which the canal ray 

 particles are moving. Accompanying the shifted lines are lines show- 

 ing no displacement, " rest " lines due to centers that are comparatively 

 at rest. The " rest " line is usually narrow while the shifted line, due 

 to rapidly moving centers, is rather wide, the violet side of the line 

 often being the sharpest. The width of the line indicates the range of 

 velocity of the canal-ray emitting centers. Making certain assumptions 

 as to the potential gradient through which the centers have passed. 

 Stark has calculated the charge carried by centers emitting the various 

 lines. 



Since the " rest " and " shifted " lines are separated by a dark 

 region. Stark concluded that canal-ray centers can only radiate line 

 spectra when their velocity exceeds a certain critical value, this critical 

 velocity increasing as the wave-length decreases. Increasing the purity 

 of the gas increases the relative intensity of the " shifted " lines. 

 Strosser has caused a stream of canal-ray centers to impinge into a cur- 

 rent of a foreign gas. The "lines of the foreign gas were found to 

 increase in intensity on leaving the cathode, pass through a maximum 

 and then decrease in intensity. The intensity of the lines of the canal- 

 ray centers decreased in intensity as the distance from the cathode 

 increased. 



Caeeiers of Spaek Spectea 



Spark spectra have been photographed on rapidly-moving films by 

 Schuster, Hemsalech, Schenck and others. The length of time the 

 metallic vapor continued to emit line spectra was found to vary from 



