356 Prof. J. J. Thomson on the 



which is equal to 



If V is equal to 1 volt and t to 10~ 8 , this is about 44. 

 Thus, if we take the cases of distilled water and an electrically 

 neutral solution of sodium chloride, which is an exceedingly 

 weak solution; then, if there were a difference of potential of 

 1 volt between distilled water and air, the apparent surface- 

 tension of the salt-solution would exceed that of pure water 

 by about 44. The effect of the electrification on the surface- 

 tension is proportional to the square of the potential-difference. 

 Thus for a potential -difference of one tenth of a volt the dif- 

 ference between the surface-tension of distilled water and a 

 weak salt-solution from this cause would only be about *44. 

 It is doubtful whether the measurements of surface-tension 

 could be trusted to detect differences as small as this; so that 

 all we can infer from the observations on the surface-tension 

 of solutions is that the difference of potential between distilled 

 water and air cannot be much greater than one tenth of a 

 volt. 



There seems no reason for limiting the possession of this 

 double coating to liquids. It is possessed by liquids of the 

 most diverse characters, as is shown by the electrification 

 developed by drops of water, mercury, molten metals, tur- 

 pentine, &c. , If, however, we suppose that solids possess such 

 a coating, it is evident that the rubbing off of part of one of 

 these coatings when two solids are rubbed together would show 

 itself as electricity developed by friction. Indeed it seems 

 quite possible that a large part, if not the whole, of the elec- 

 tricity developed by friction may be due to this cause. 



Another phenomenon in which I am inclined to think this 

 double layer of electrification over the surface of bodies plays 

 an important part, is the electrification of metals and fluor- 

 escent liquids when exposed to the influence of ultra-violet 

 light. These substances under such circumstances acquire a 

 charge of positive electricity, the negative electricity going to 

 the air. Now in mercury and molten liquids and solutions of 

 the fluorescent substances eosine and fluorescene the positive 

 layer of the double coating is next the metal or liquid, the 

 negative layer next the air. It seems quite possible that 

 when there is intense reflexion of light from the surface of 

 these substances, the outer coating may get partially dissi- 

 pated, leaving the metal or liquid with a positive charge. A 

 solution of rosaniline also shows the same effect when 

 exposed to ultra-violet light, though the electrification of its 

 drops' is at low temperatures of the opposite sign to that of 





