Photo-Electrons are emitted from Matter. 685 



appears, therefore, that the application of the quantum theory 

 to photo-electricity is quantitatively justified if we keep in 

 mind the electrons which emerge in the direction of the 

 incident light. 



No experimental results as to the ratios of the emergent 

 to the incident velocities for other metals are available, and 

 so it is not possible, at present, to test this view further. 

 More precise information would be obtained if the ratios 

 were investigated using monochromatic light. 



From equation (1) it is seen that there can be no emission 

 of electrons unless li'n exceeds V e. On putting V = we 

 get the critical frequency at which the electrons emerge 

 without any velocity. Below this frequency there is no 

 photo-electric effect. Knowing the critical frequency at 

 which the photo-electric effect starts, we may calculate Y e, 

 the amount of energy required to take an electron from the 

 molecule, and V may be regarded as the ionizing potential. 



We shall now make the assumption that the ionization 

 of gases by ultra-violet light is essentially the same as the 

 photo-electric effect in solids, and we may proceed to find 

 the ionizing potential of a gas from the longest wave-length 

 which produces ionization in it. A difficulty arises as to 

 what value to assign to It for a gas, since h' is only obtained 

 from velocity experiments with solids, and varies slightly 

 from one element to another. The difficulty disappears if 

 we consider that Robinson's experiments, in conjunction with 

 those of Richardson and Compton, justify us in regarding 

 equation (2), and net equation (1), as representing the 

 energy exchanges associated with the emission of photo- 

 electrons from isolated molecules of a gas. 



It has been shown by the writer * that the ionization of 

 air by ultra-violet light sets in at about A, 1350. (From a 

 consideration of Lenard's work, Lyman f concludes that the 

 critical wave-length is about X 1300, which agrees well with 

 my results.) Palmer's work + leads us to associate this 

 critical wave-length with the oxygen in the air. If we take 

 h' for oxygen to be 5*72 x 10~ 27 , which is the mean value 

 for the metals, as an approximation, we get V to be 8*0 volts. 

 This method of calculating V was introduced in a previous 

 paper. But the considerations brought forward earlier in 

 this paper now justify us in using A instead of It . Taking 



* Hughes, Proc. Camb. Phil. Soc. xv. p. 482 (1910). The outside 

 limits given in the paper are X1250 and X1450. The critical -ware- 

 length X 1350 was obtained from subsidiary experiments in which tluorite 

 plates possessing different transparency limits were used. 



t Lyman, Phys. Zeits. xiii. p. 583 (1912). 



X Palmer, Phys. Rev. xxxii. p. 1 (1911). 



