LIGHT AND ELECTRIFICATION. 179 



If the surfaces are very clean, and if the light is ordinary 

 ultra-violet light, the difference between the rates of dis- 

 charge of negative and positive electrification is so enormous 

 that it is practically true to say that one pours away while 

 the other is retained ; but more careful observation will 

 show that the rate of leakage of positive, too, is usually 

 increased by illumination. The leakage of positive, how- 

 ever, occurs better from surfaces not too recently cleaned, 

 and it appears as if still shorter wave lengths were necessary 

 for it than for leakage of negative. 



My experiments go to show that as metals get dirty 

 the rates of leakage of negative and positive become more 

 comparable. And one metal I have found which is able to 

 discharge positive even faster than negative, vis., cobalt. 

 What the meaning of this exceptional behaviour may be 

 is quite unknown, but the general rule seems to be that the 

 discharging order of the metals follow the voltaic series, 

 and that the more oxidisable metals discharge negative 

 more easily, and under longer wave lengths, than do the 

 less oxidisable. Righi further finds that the tendency of 

 metals to become positively charged under the action of 

 light likewise follows the voltaic order, but inversely ; gold 

 and carbon being more effective in acquiring a positive charge 

 than zinc and aluminium. It may be doubted whether this is 

 capable of thorough substantiation as a general law or not, 

 but there can be no doubt that the discharge of negative 

 electricity is most rapid from perfectly clean metals ; — they 

 need not be polished, scratching with sandpaper suffices, 

 but a few minutes' exposure to air destroys the initial 

 extreme vigour of the effect. It is not true that a positive 

 charge is wholly retained under the action of light even by 

 such clean metals ; they do discharge it, but very much 

 more slowly than they do a negative charge ; whereas when 

 they are tarnished the rate of discharge for the two kinds of 

 electrification is more nearly equal, and when they are dusty 

 or dirty it is quite possible for positive electricity to be dis- 

 charged more rapidly than negative by the action of light. 

 The following extracts from my laboratory note-book may 

 serve to establish and illustrate this fact : — 



