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XXXIII. The Photo-Electric Effect of some Compounds. 

 By A. Ll. Hughes, D.Sc, B.A., Research Scholar of 

 Emmanuel College, Cambridge*. 



OST of our knowledge of the photo-electric effect has 

 been derived from experiments on metallic surfaces. 

 Comparatively few experiments have been carried out on 

 surfaces of salts and other compounds. Some early experi- 

 ments were carried out on compounds (chiefly salts) by 

 Knoblauch f. Stark J investigated the photo-electric effect 

 of a number of organic substances. By k and Borck § studied 

 the photo-electric effect of solid anthracene. In a paper ||, 

 which deals mainly with an attempt to find ionization by 

 ultra-violet light in vapours containing photo-electric elements, 

 a few experiments are described on the photo-electric effect of 

 certain solid compounds. It was concluded that the surfaces 

 prepared by dipping a metal plate into a molten salt and 

 allowing a layer to solidify on the plate were very un- 

 satisfactory. As the photo- electrons come from a depth of 

 only a few molecules, it is essential that the outermost surface 

 layer should be chemically identical with the interior, if we 

 are to consider that we are studying the real photo-electric 

 effect of any particular salt. The most satisfactory way of 

 preparing a pure surface of any salt or other compound is to 

 distil it in a liquid air vacuum. The surface layers may then 

 be considered to be chemically identical with the rest of the 

 deposit. To investigate such surfaces, I used the apparatus with 

 which I measured the emission velocities of photo-electrons 

 from surfaces of metals prepared by distillation in vacuo %. 

 The experiments showed that it required more energy to 

 liberate an electron from an electronegative element like 

 arsenic or selenium, than from metals. If a metal be in com- 

 bination with an electronegative element, then more work will 

 be required to liberate the electron from the molecule than 

 from the uncombined metallic atom. It was thought that 

 quantitative measurements of the emission velocities of photo- 

 electrons from, salts would furnish very useful information 

 regarding the work necessary to liberate an electron from 

 the molecules of different compounds. Such results, if they 

 could be obtained, would be of the utmost value in forming 

 an electrical theory of chemical affinity. The investigation, 



* Communicated by Sir J. J. Thomson, O.M ., F.K.S. 

 t Knoblauch, Zeii.f. Phys. Chetoiie, xxix. p. 527 (1899). 

 \ Stark, Phys. Zeits. ix. pp. 481, 661 (1908). 

 § Byk & Borck, Verh. d. D. Phys. Ges. p. 621 (1910). 

 I| Hughes, Proc. Camb. Phil. Sbc. xvi. p. 375 (1911). 

 IT Hughes, Phil. Trans. (A), vol. ccxii. p. 205 (M-2), 



