It acted by induction upon the concave surface of the latter, and he examined how this act of induction was effected by placing insulators of various kinds between the two spheres.

He tried gases, liquids, and solids, but the solids alone gave him positive results.

He constructed two instruments of the foregoing description, equal in size and similar in form.

The interior sphere of each communicated with the external air by a brass stem ending in a knob.

The apparatus was virtually a Leyden jar, the two coatings of which were the two spheres, with a thick and variable insulator between them.