104 Mr. J. Swinburne 



on 



falls, there is an extra current produced which is not propor- 

 tional to the rate of increase of the electromotive force, and 

 this supplies the loss in the condenser. The condenser may 

 still show perfect insulation under a direct pressure. Accord- 

 ing to this theory, absorption is necessarily accompanied by 

 an increase of capacity. This is important in connexion with 

 the electromagnetic theory of light. Absorption would 

 always increase the apparent capacity of a condenser, so that 

 determinations of specific inductive capacity of absorptive 

 dielectrics would come out too high. The dielectrics which 

 come out too high, such as glass, are just those which heat 

 most. The energy absorbed in a given dielectric would be a 

 function of the frequency and the resistance of the conducting 

 parts. It is thus possible that a dielectric might be opaque 

 to light, and diathermanous if the resistance is comparatively 

 low, or transparent to light, but not to dark heat if it is some- 

 what higher. Similarly a Leyden jar, which absorbs power 

 when connected to an alternating dynamo, may lose little of 

 its energy internally when discharged oscillatorily by a spark. 

 The case of a fibrous imperfect insulator embedded in a 

 perfect insulator is not susceptible of mathematical treat- 

 ment and was not taken by Maxwell. Mica is perhaps a 

 better example. It is a good insulator, but gets very hot. 

 If a thin plate is put between two sheets of tinfoil, heating 

 soon shows. Here we have tinfoil, then a stratum of air, then 

 mica, which is itself stratified, then air, then tinfoil again. 

 The static attraction presses the foil close against the mica, 

 still there is air. Unless the mica is very thick, the fall of 

 potential between the plates is so rapid that the air breaks 

 down and there is a disruptive discharge between the foil and 

 the mica. This actually occurs ; there appears to be a lumi- 

 nous layer of minute blue sparks under the foils, and there is 

 a strong smell of ozone. The discharge under the foils, which 

 is rendered visible by the transparency of the mica, must, not 

 be confused with the brush-discharge round the edges of the 

 foil, which differs in appearance. There is considerable heat- 

 ing even when the foil is affixed with paraffin-wax so that 

 there is no air. 



Of course mica, being stratified, may consist of alternate 

 sheets of some imperfect conductor and an insulator. It is 

 more difficult to find a reason why glass should heat. At 

 high temperatures it is an electrolyte, and it has no definite 

 solidifying-point, so it may be partially electrolytic at ordi- 

 nary temperatures. Such an explanation can hardly be 

 applied to such a substance as celluloid. This is a very per- 

 fect insulator, at least till it breaks down disruptively, and it 



