APPENDIX. 563 



and in the medium 2 the pressure 



P2 = 8^ S2jP2) 

 in the opposite direction. 



There is accordingly on the element of surface the difference of 

 pressure 



(7) P = 



acting towards the medium 1. If the two media lie side by side 

 between the plates of a plane condenser we have F the same in 

 both media, so that the pressure is greater in the medium for 

 which fji is greater, and the surface is impelled towards the other 

 medium. This has been verified by Quincke*, who blew a large 

 bubble of air into a liquid contained between the plates of a con- 

 denser, and observed the additional pressure necessary to be given 

 the air in order to resist the pressure due to the fluid. Quincke 

 also verified the tension in the direction of the lines of force by 

 filling an absolute electrometer with liquid. The stresses in 

 magnetized media have been similarly verified by experiments by 

 Quincke *( and Taylor Jones J. 



Upon the principle of the sidewise force Mr A. P. Wills has 

 founded an accurate method for determining K for substances in 

 which it is extremely small, both for magnetic and diamagnetic 

 substances, by observing the attraction or repulsion on a slab with 

 its edge in a uniform field. 



* Quincke, " Electrische Untersuchungen," Wied. Ann. xix. p. 705, 1883. 



t Ibid. xxiv. p. 347, 1885. 



J Jones, " On Electromagnetic Stress," Phil. Mag. xxxix. p. 254, 1895. 



362 



