Photo-Electric Effect of some Compounds. 387 



clearance of 1 or 2 mm. With different samples of distilled 

 water, leaks of from 5 to 15 units were obtained. In this 

 case, owing to the bigger aperture and the fact that the lamp 

 could be placed closer to it, the amount of light which fell 

 on the surface of the water was 7 or 8 times greater than that 

 which fell on the surfaces of the salts. 



The variations in the leak seem to indicate that the leak is 

 more likely due to some spurious effect than to a real effect 

 with the pure water. It is impossible, with this simple 

 arrangement, to obtain the same clearance between the rings 

 and the water surface time after time. Hence, the amount 

 of stray light which gets to the edges of the dish may vary 

 to a great extent. It is also possible that slight traces of 

 impurities in the water may give rise to photo-electric effects. 

 To prove that water is not photo-electric to a higher degree 

 of accuracy than w 7 as obtained in these experiments would 

 require more elaborate experimental arrangements. However, 

 it seems clearly established that the photo-electric properties 

 of the water in the surfaces of the moist ZnCl 2 and P 2 5 are 

 quite inadequate to account for the big emission of electricity 

 from these surfaces. The effect seems to depend, in son e 

 way, on the combination of the water with the ZnCl 2 and 

 theP 2 5 . 



Anthrac 



ene. 



Stark * found that anthracene vapour could be ionized by 

 means of the light from a mercury lamp. Anthracene has 

 also been shown to be photoelectric when in the solid state, 

 Anthracene is one of the few organic compounds suitable for 

 experiments on the emission velocities of its photo-electrons, 

 since it has no appreciable vapour pressure at ordinary 

 temperatures to interfere with the velocity measurements. 

 The total leak from the anthracene surface was much larger 

 than that from any of the compounds previously used, being 

 of the order 10 3 units. When illuminated by the unresolved 

 light from the mercury arc, the anthracene charged up to a 

 potential of 87 volt. The great photo-electric sensitiveness 

 of anthracene enabled one to study its effect with isolated 

 wave-lengths. There was not the slightest trace of any 

 effect with wave-lengths longer than X2200. Well-marked 

 effects were obtained with the lines between XI 849 and 

 X2002. On account of the much greater energy in the lines 

 X2257 and X2536, the photo-electric effect for metals with 

 these lines was many times greater than the effect with 

 X1849. This shows clearly that the effect for anthracene 

 * Stark, Phys. Zeit. x. p. 614 (1909). 



