Instantaneous spreading of charges 



'59 



air in the form of a spark. This is visible in a dark room, as one may see 

 a faint light on the surface of the glass all round the edges of the coating, 

 especially if the glass is thin, for if it is thick it is not so visible*. 



[From a photograph taken in the Cavendish Laboratory of a plate of glass with 

 a circular tinfoil coating on one side, a larger coating being applied to the other side 

 of the glass. The electrification of the coatings was produced by an induction coil.] 



308] There is another circumstance which shews this instantaneous 

 spreading of the electricity, namely, after having charged and discharged 

 a coated plate of glass a great many times together without cleaning it, 



* [See Art. 532, Feb. i, 1773.] 



