HISTORICAL. 293 



and differently for different lines. Langenbach' obtained similar re- 

 sults for the visible spectrum, except that with constant current the 

 intensity passes through a maximum for decrease in pressure of the 

 gas. He found the three prominent hydrogen lines most intense for 

 a pressure of about 2.5 mm. to 3.5 mm., and that they are very similar 

 in their behavior with change in current, which was obtained from an 

 induction coil. With increase in pressure the intensity of the red line 

 increases the most; the energy maximum shifts toward the long wave- 

 lengths, /. c, hydrogen behaves like a black body. 



This work was repeated by Berndt," who verified the work of Ferry, 

 and added a new result, viz, that a rise in temperature of 200° has no 

 influence upon the individual lines. For hydrogen he found that the 

 green line grows more rapidly in intensity than does the red one, while 

 for nitrogen the reverse is true of the red and green bands, which dis- 

 proves the displacement law for gases. 



This work was continued by Waetzman,^ who verified the work of 

 Ferry and Berndt, and showed that for a slight impurity the gas be- 

 haves as the pure, but this impurity decreases the intensity of the long 

 wave-lengths the most. 



Temperature, Dissociation, etc. 



The explanation of the luminous radiation from a vacuum-tube which 

 is itself quite cool, has received considerable attention. Wiedemann,* 

 using a calorimeter, shows that the average temperature of air in a 

 vacuum-tube, at a pressure less than 3 mm., is less than 100° C, and that 

 it depends upon the pressure. At a low pressure the temperature of the 

 cathode is higher than that of the anode, while the reverse is true at a 

 pressure greater than 2.6 mm. i\lthough the average temperature is 

 low, few molecules may possess an amount of kinetic energy much 

 greater than that indicated by the bolometer. Naccari and Gugliemo" 

 found that at a pressure of 5 mm. the cathode is the hotter, reaching 

 a maximum at 0.3 mm., then decreasing until at very low pressure, 

 where the anode is the hotter. They found this to be due to a secondary 

 action coming from the cathode rays. 



Wiedemann (loc. cit.) does not consider the radiation from the gas 

 in a vacuum-tube to be due to a pure thermal effect, but of a phosphor- 

 escent nature. He applies the term luminescence to all those luminous 



^ Langenbach : Ann. der Physik (4), 10, p. 789, 1903. 

 ^Berndt : Ann. der Physik (4), 12, p. iioi, 1903. 

 ^Waetzman: Ann der Physik (4), 14, p. 772, 1904. 



* Wiedemann ; Ann der Physik (3), 6, p. 298, 1879 ; 7, p. 500, 1879 ; 9, p. 150, 1880 ; 

 10, p. 202, 1880. 



* Naccari & Guglielmo : Nuovo Cimento (3), 15, p. 272, 1884. ' 



