Complete Photoelectric Emission. 155 



are those observed by H. A. Wilson* with well oxidized 

 wires. He found 4x 10" 8 amp. per cm. 2 at 1686° C, which 

 corresponds to nearly 10~ 7 amp. per cm. 2 at 2000° K. Even 

 in this extreme case the observed thermionic current is 

 about 5000 times as large as the complete photoelectric 

 current. 



We are thus led to the conclusion that the complete 

 photoelectric emission gives rise to an insignificant portion 

 only o£ the observed thermionic currents. While this con- 

 clusion cannot be held to be established with absolute certainty, 

 on account of the doubtfulness of some of the factors entering 

 into the calculations, it does, nevertheless, appear extremely 

 probable on the evidence. In view of its importance, it is 

 desirable that the question should be settled quite definitely; 

 but it is questionable whether any considerable advance on 

 the present calculations can be effected without a material 

 extension of our knowledge of the conditions involved in 

 photoelectric action. 



If photoelectric action had proved successful in accounting 

 completely for thermionic emission of electrons, one would 

 have been tempted to speculate further, and to ask whether 

 ordinary evaporation — the emission of uncharged molecules 

 instead of charged electrons — might not be attributable in 

 a similar way to the direct action of radiation. One could 

 imagine the emission of molecules in evaporation to be the 

 result of a kind of photo-chemical action, the effective 

 radiations being mainly in the infra-red. Such a position 

 would obviously be in line with current ideas as to the nature 

 of heat-energy which are based on the quantum hypothesis. 

 It is not improbable that certain radiations may be found to 

 have a selective and direct effect in increasing evaporation, 

 in addition to the quite obvious bat less direct effect due to 

 the heat generated by the absorption of the radiation ; and 

 this effect is worth looking for. But judging from the 

 analogy with the relation between ordinary photoelectric 

 action and the emission of electrons from hot bodies, it 

 seems likely that such effects, if they exist, will account 

 for only a small fraction of the emission of molecules in 

 ordinary evaporation. 



* Phil. Trans. A. vol. ccii. p. 262 (1903). 



M2 



