Prof. Gr. Quincke on Electrical Expansion. 37 



It is remarkable that the order of the substances examined, 

 whether they be arranged according to thermic expansion or 

 electric expansion, has no relation to the order of their con- 

 ductivities for electricity. 



34. Changes of temperature of a few hundredths of a 

 degree suffice to bring about the change of volume produced 

 by electricity in the first group of substances, that to which 

 water belongs. 



The length of time that the expansion lasts in the case 

 of insulating fluids is opposed to the hypothesis of an in- 

 direct expansion, resulting from heat produced by feeble 

 currents of electricity between the electrodes — as is also the 

 small increase of the expansion when the conducting-power 

 of water is increased by the addition of hydrochloric acid, and 

 also the decrease in volume in the case of the fatty oils, which 

 also expand upon increase of temperature from 0° C. 



Electric Perforation of Glass. 



35. Electric force produces generally the same effect as 

 heat, namely expansion. But as by unequal application of 

 heat to different portions of a body it may be broken, so also 

 its fracture may be brought about by the unequal action of 

 electric force. 



Uniform expansion by heat or by electric force is not 

 capable of fracturing glass ; but unequal thermic or electric 

 expansion does so, and the more readily the greater the elastic 

 tension produced in the interior of the glass. 



What is true of glass is true of other substances. Thick 

 masses, and such as conduct heat or electricity badly, are 

 fractured more easily than thin masses, or such as conduct 

 heat or electricity readily. This is confirmed by experiment. 



Electric Double Refraction. 



36. It is well known that by unequal application of heat 

 solid transparent substances are unequally expanded, and 

 become optically double-refracting. 



In the same way, by unequal electric expansion, substances 

 become optically double -refracting. This explains the double 

 refraction observed by Kerr *, which glass, quartz, resin, and 

 insulating fluids exhibit under the influence of electricity, and 

 the apparent contradiction of these results with those obtained 

 by other observers. 



If long thin glass plates are coated with tinfoil and power- 



* Phil. Mag. (4) 1. pp, 337-348, 446-558, 1875 ; ib. (5) viii. pp. 85- 

 102, 229-245, 1879. 



