148 Dr. Hopkinson. Electrostatic Capacity of Glass. [Dec. 9. 



wood, which in the arrangement of its vessels and medullary rajs 

 resembles so closely the genns Platanus that it most probably is 

 P. acerdides, which occurs at Mackenzie River in the leaf beds. 



More complete details of my investigation will be found in my 

 paper on the " Fossil Woods of the Arctic Regions," in the forth- 

 coming volume (VI) of the " Flora Fossilis Arctica," by Professor 

 Heer. 



III. " The Electrostatic Capacity of Glass." By J. Hopkinson, 

 M.A., D.Sc, F.R.S. Received November 3, 1880. 



(Abstract.) 



In 1877 I had the honour of presenting to the Royal Society* the 

 results of some determinations of specific inductive capacity of 

 glasses, the results being obtained with comparatively low electro- 

 motive forces, and with periods of charge and discharge of sensible 

 duration. In 1878 Mr. Gordonf presented to the Royal Society results 

 of experiments, some of them upon precisely similar glasses, by a 

 quite different method with much greater electromotive forces, and 

 with very short times of charge and discharge. Mr. Gordon's results 

 and mine differ to an extent which mere errors of observation cannot 

 account for. Thus, for double extra dense flint glass I gave 10" 1, 

 Mr. Gordon 3*1, and subsequently 3*894 These results indicate one 

 of three things, either my method is radically bad, Mr. Gordon's 

 method is bad, or there are some physical facts not yet investigated 

 which would account for the difference. Two possible explanations 

 have been suggested : 1st, possibly for glass K is not a constant, but is 

 a function of the electromotive force. 2nd. When a glass condenser is 

 discharged for any finite time, a part of the residual discharge will be 

 included with the instantaneous discharge, and the greater the time 

 the greater the error so caused. To test the first I measured the 

 capacity of thick glass plates with differences of potential ranging 

 from 10 to 500 volts, and also of thin glass flasks between similar 

 limits ; the result is that I cannot say that the capacity is either greater 

 or less where the electromotive force is 5,000 volts per millimetre than 

 where it is J volt per millimetre. The easiest way to test the second 

 hypothesis is to ascertain how nearly a glass flask can be discharged 

 in an exceedingly short time. A flask of light flint glass was tested ; 

 it was charged for some seconds, discharged for a time not greater 

 than lyo oo secon d, and the residual charge observed so soon as the 



* " Phil. Trans.," 1878, p. 17. 



f "Phil. Trans.," 1879, p. 417. 



X "Report of British Association," 187^. 



