142 Roy a I Society : — 



If t be small, 



J.'=-X^(T), 



rand the value of — -' may be observed with more or less accuracy. 

 2ud. Let o;^ be constant = X for a very long time T previous to 

 time c=0 ; discharge and at time t insulate and observe -^. 



'T+f 



-"-1 



iJ'=x{f(0-B}. 



clt 



There are also methods of verification. Tor example, charge 

 during time T', reverse the charge for time T" and discharge ; then 



after time t insulate and observe -— -^ ; we shall find 



dt 



III. Experiments were tried on ten glasses. The verifications 

 were perhaps as close as could be expected, considering that no 

 attempt was made to observe at a constant temperature. The 

 glasses were : — 



No. 1. A soda-lime glass containing much soda. 



No. 2. A soda-lime glass coloured deep blue with oxide of cobalt. 



No. 3. Window-glass. 



No. 4. Optical hard crown. 



No. 5. Soft crown. 



No. 6. Avery light flint glass. 



No. 7. Light flint. 



No. 8. Dense flint. 



No. 9. Extra-dense flint. 



No. 10. Opal glass. 



Glasses 1, 2, and 3 agree in possessing high conductivity and 

 also large values of yj/^t—'B ; whilst 7, 8, 9, 10 have a high resis- 

 tance (thousands of times as great as 1, 2, or 3) and small residual 

 charge. 



IV. Electrolytic conduction may occur through the soda-lime 

 glasses at the ordinary temperature of the air. 



Summary. — The experiments appear to verify the fundamental 

 hypothesis, viz. that the effects on a dielectric of past and pre- 

 sent electromotive forces are superposable. Ohm's law asserts 

 the principle of superposition in bodies in which conduction is 

 not complicated by residual charge. Conduction and residual 

 charge may be treated as parts of the same phenomenon — an after 

 effect, as regards electric displacement, of electromotive force. 

 The experiments appear to show that the principle of Ohm's law 

 is true of the whole phenomenon of conduction through glass. 



