314 Royal Society. 



the mill had become constant, the vessel returned to its original 

 position. On suddenly cutting off the light the vessel was again 

 deflected, but in the opposite direction to that on starting the experi- 

 ment. The vessel therefore now turned in the same direction in 

 which the mill turned. 



These experiments are easily explained on the assumption that the 

 force acting on the vessel enclosing the light-mill is exactly equal 

 and opposite to that acting on the mill itself. While the velocity 

 of the mill in one direction is increasing, a force acts in the oppo- 

 site direction on the vessel. When the velocity has become con- 

 stant, the force which tends to drive the mill round is exactly 

 counterbalanced by the resistance which opposes the motion of the 

 mill. The two forces acting on the vessel will therefore counter- 

 balance, and the vessel will return to its original position of rest. 

 When the light is cut off, the resistance will stop the motion of 

 the mill. The reaction of the resistance will act on the enclosure, 

 and the enclosure will turn in the same direction as the mill. 



By means of the reaction on the enclosure I have been able to 

 calculate the strength of the force; and I have found that the pres- 

 sure on a surface on which light of equal intensity to that used in 

 my experiments falls, is equal to that produced by the weight of 

 a film of water on a horizontal surface equal in thickness to the 

 length of a wave of violet light. 



March 30.— Dr. J". Dalton Hooker, C.B., President, in the Chair. 



The following paper was read : — 



" The Besidual Charge of the Leyden Jar." By J. Hopkinson, 

 M.A., D.Sc. 



1. If it be assumed that a dielectric under electric induction has 

 every element of volume of its substance in an electropolar state, 

 and also that dielectrics have a property analogous to coercive force 

 in magnetism whereby time is requisite for the development or de- 

 cay of this electropolar state, an explanation of the residual charge 

 of the Leyden jar easily follows. Adding the hypothesis, which at 

 first sight appears most probable to connect the induction and 

 polarization with the time by two differential equations, it follows 

 that the potential E of a Leyden jar when insulated may be ex- 

 pressed in the form E=(A -j-Be-^)e -A *, where X and p are con- 

 stants for the material, and A and B depend on the previous states 

 of the dielectric. 



2. Observations made with the quadrant electrometer, the con- 

 denser being a Florence flask containing sulphuric acid, shows that 

 E cannot be so expressed. Glass is a mixture of different silicates, 

 and it may be supposed that each substance is capable of indepen- 

 dently being electropolar ; there will thus be E and more than one 

 polarity to be connected with the time by more than two differential 

 equations. Making a similar obvious hypothesis regarding these 



r ■ 



relations, E must be expressed in the form 2 A^e - ^. If this be 







so, it would probably be possible to charge a Levden jar in such a 



