OIS ELECTROLYSIS IN ITS PHYSICAL AND CHEMICAL BEARINGS. 309 



that for very much greater forces tbese substances do not obey Ohm's law : the 

 departure from it being very marked. They also find that the conductivity is 

 improved by raising the temperature. A full account of these experiments is to 

 be communicated to the Royal Society shortly. 



On Continuity of Electric Conduction. By Dr. John Hopkinson, F.B.S. 



Jn my experiments on residual charge I touched upon the second question in 

 Dr. Lodge's programme ('Is Ohm's law obeyed by very bad conductors?')/ and 

 pointed out that Ohm's law could be regarded as a limiting case of a more general 

 law of superposition. In the case of mechanical after-effects the law of super- 

 position does not hold even approximately. The fourth question (' Is there any 

 relation between optical opacity and electrolytic conductivity ? ') appears to me 

 to be very intimately associated with the fact that bodies which, if they conducted, 

 would be electrolysed do not follow Maxwell's law, whereas some other insulators 

 do. My own present impression is that an electrical displacement in glass may, 

 although continuous, be roughly divided into four successive stages. 1st. A. yield- 

 ing of the dielectric during a time corresponding to the time of wave-frequency of 

 light, for which K = 2i about. 2nd. A fiu'ther yielding during a time correspond- 

 ing to great absorption below the red, bringing K up to from 6 to 10. 3rd. A 

 further slow yielding, partly recoverable, hardly sensible in time less than a second 

 or such like, and going on with diminishing amount for days. 4th. A yielding 

 coiTesponding to an actual decomposition of the material. Superposition probably 

 applies to all these continuously connected successive events. Probably if we could 

 experiment fast enough on any ordinary electrolyte, like solution of OUSO4, we 

 should find a similar succession of phenomena. 



[Dr. Hoj^kinson's note is of extreme interest, and the references to his papers are 

 Its follows : Residual Charge in Leyden Jar, ' Phil. Trans.' January 1877 ; Strain 

 in Glass Fibre, ' Proc. Roy. Soc' October 4, 1878 ; Refractive Index and Specific 

 Inductive Capacity, 'Phil. Mag.' April 1882. This last paper I may abstract 

 thus: — 



Maxwell's laws are that /x" = K, and that transparent bodies must insulate. 

 They are true for mineral oils and solid paraffin ; not true for glass, Iceland spar, 

 and organic oils. Consider, for instance, light flint glass : K is 67 for disturbances 

 whose period is longer than 10"^ second, and for these disturbances it behaves as 

 an insulator. It ought, therefore, for such waves to be transparent, and to have an 

 index 2-6. But, for disturbances of period about 10"'* second, its index, reckoned 

 for very long waves by extrapolation formula, comes out about 1 -5. Is there any 

 way of accounting for this discrepancy ? Yes ; perhaps by the known fact that on 

 waves between these two periods glass exercises a strong selective absorption, and 

 that this is usually accompanied by anomalous dispersion ; which at once renders 

 all empirical reasoning towards the state of things for very long waves, from the 

 observed condition for very short waves, utterly futile and misleading. Perhaps, 

 therefore. Maxwell's law is after all obeyed by these substances for long waves ; and 

 one way to test the question is by using rays from a thermopile to a freezing 

 mixture. 0. L.] 



On Diathermancy and Electrolytic Conductivity. 

 By Shelford Bidwell, F.B.S. 



The following is one of the questions suggested by Dr. Lodge for the considera- 

 tion of the Committee on electrolysis : — Is there any relation between optical 

 opacity and electrolytic conductivity ? ^ 



Assuming that ' optical opacity ' is included in the more comprehensive term 

 -* opacity to radiation,' I have endeavoured to ascertain experimentally whether 



' See Brit. Assoc. Rejjort for 1885, p. 765. 

 ^ IhlA. p. 768. 



