206 HENKY A. EOWLAND 



fectly homogeneous. Thus mica and selenite are so very lamellar in 

 their character, that few specimens ever appear in which the lamina 

 are not more or less separated from one another; and thus they should 

 have electric absorption. 



II 



In the ordinary method of experimenting with the various forms of 

 Leyden jar, there are, besides the residual discharge due to electric 

 absorption in the substance of the insulator, two other sources of a 

 return charge. The surface of the glass being more or less conduct- 

 ing, an electric charge creeps over the surface from the edges of the 

 tinfoil. In discharging the jar in the usual way by a connecting wire, 

 this surface remains charged, and the electricity is gradually con- 

 ducted back to the coatings, and thus recharges them. If, further- 

 more, the coatings be fastened to the glass with shellac or other cement, 

 the return charge may be partly due to it; for we have between the 

 coatings not merely glass, but layers of glass, cement, &c., which the 

 theory shows to give a residual discharge. Besides the coatings are 

 not planes; and hence, as one of us has shown, there may be a return 

 charge, even if the glass gave none between infinite planes. If the 

 plates were merely laid on the glass without cementing, the same 

 result would follow, since the insulator would then consist of air and 

 glass in layers. 



In the present research these were sources of error to be avoided, 

 since the residual discharge due to the insulating plates themselves 

 were to be compared. The condenser-plates were copper disks. These 

 were amalgamated, so that there was a layer of mercury between them 

 and the dielectric, which excluded the air and conducted the electricity 

 directly to the surface of the dielectric : thus the condition of a single 

 substance between the plates was fulfilled. The errors due to the 

 creeping of the charge over the surface of the dielectric and that due 

 to the plates not being infinite were avoided, the first entirely and the 

 second partially, by the use of the guard-ring principle of Sir Win. 

 Thomson. 



Plate IV represents this apparatus. The plate of crystal, c, was 

 placed between two amalgamated plates of copper, a and &, over the 

 upper one of which the guard-ring, d, was carefully fitted; this ring, 

 when down, served to charge and discharge the surface around the 

 plate, a; and so the errors above referred to from the creeping of the 

 charge along the plate, and from the plate not being infinite, were 

 avoided. 



