208 Dr. L. T. More on the Supposed Elongation of 



especially noticeable in Quincke's method, which was to 

 stretch fine glass fibres horizontally between two supports. 

 He also gives prominence to the fact, that if a bent glass 

 tube, or one not uniformly thick, be used there is a large 

 amount of distortion due to the non-uniformity of the field. 

 This irregularity must also occur when thermometers are 

 employed, as it is naturally difficult to blow a bulb perfectly 

 spherical, and also everywhere uniform in thickness. Quincke, 

 on page 190, states, " that thermometer No. 34 had a bulb 

 of somewhat irregular shape and wall thickness, so that the 

 inner portions of the bulb on electrification would be especially 

 affected. It is therefore not comparable with the others, and 

 is included only because it shows the greatest change of 

 volume which I have been able to observe in glass/' This 

 thermometer showed an expansion of 68'36 where No. 9 of 

 the same glass, of practically the same thickness and with 

 the same difference of potential, gave only 5'277. This great 

 difference due to irregularity of shape makes one wonder 

 whether the expansion of No. 9 might not partly result from 

 the same causes. Also heating effects would be less evident 

 in my experiments as the thicker glass and lower potential 

 gradients employed would reduce this effect. 



From the results of my experiments, taken with the con- 

 flicting evidence of former investigators, it is not probable 

 that the stress occurring in the aether when a dielectric is 

 electrostatically charged causes a mechanical deformation of 

 the substance. At least if there is such an action it must be 

 extremely minute — perhaps a slight rearrangement of the 

 molecular structure — as an effect so large as one expressed by 

 the formula KH 2 /87r could not have escaped my notice. 

 Though it might have been expected that such sethereal 

 stresses would change the length of a dielectric botn along 

 and at right angles to the lines of induction, yet as one of 

 these is a tension and the other a compression of equal amounts, 

 it is difficult to see how any change of volume other than a 

 differential effect would result, whereas Quincke finds that 

 the change of volume is nearly three times as large as the 

 linear expansion. 



In conclusion, the evidence that the stress in the sether 

 denoted by Maxwell's formula KH 2 /8tt is accompanied by a 

 mechanical strain resulting in the deformation of the charged 

 substance, is also not verified by experiments in allied fields 

 of investigation. We may not infer from the Kerr effect of 

 dielectric double refraction that such a mutual action exists, 

 since the Kerr effect may be either positive or negative. 

 Some dielectrics, as glass, give an effect equivalent to a 



