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It has been shown by Dr. Hopkinson that, if two Leyden jars be made of the same glass, but of different thicknesses . 1st. If they be charged with the same difference of potential for equal times, discharged for equal times, and then insulated, that the residual charge will, after equal times, have in both cases the same difference of potential. 2nd. That residual charge is proportional to exciting charge. These propositions may be included in one law the superposition of simultaneous forces is applicable to the phenomena of residual charge. All the investigations in Dr. Hopkinson’s paper in the “ Transactions of the Royal Society,” vol. 167, part 2, serve to^prove this law, and, so far, they support the theory of residual charge, which we owe to the late Professor Clerk Maxwell. We should, therefore, be inclined to think that one of the best methods of investigating the relation between the relative powers of different glasses to possess residual charge would be simply to charge jars made of these glasses for the same great length of time, discharge for the same short intervals, and insiilate, measuring in each case the time increase of soaking out of the residual charge. Whatever the thickness of the glass or the amount of the original charge, we know that the same glass will always give the same proportion of residual charge at the same times from insulation. Any change of the state of the glass caused by heat would be shown as a change on the curve of increasing residual charge. There seems to be no doubt that this method would give what may be called a measure of the specific power of producing residual charge phenomena of the glass experimented on.

A preliminary account of the reduction of observations on strained material, leyden jars, and voltameters