47

importance in preparing the mind for reasoning onthe modus operandi of Galvanic action generally, Ishall repeat it in this place. The apparatus requiredfor the performance of this experiment are an elec-trical machine, a small Leyden jar, a flat cake ofpitch, a small tin can with a glass stem for insulation,and a spring puff or hag, containing a mixture ofsulphur and red lead.

The resinous cake being quite dry and free fromdust, I place the little can upon it at some short distancefrom the middle of the surface. I then electrize theinside of the jar negatively, either by placing its knobin connexion with the negative conductor of themachine, or by holding its outer coating against theprime conductor: (the former way is preferable,because the hand has already hold of the body ofthe jar) which done, I present its ball to the can,from which it recovers a small quantity of its lostfluid. The can, consequently, becomes electro-negative, and its rim in connexion with the resinouscake renders a ring of the latter also negative.

I now lift up the can by its glass handle, andfor a moment touch it with my finger to restore itselectric equilibrium. The can is then placed onanother part of the resinous surface, the jar dis-charged, and again recharged with its ball at theprime conductor. A spark is now transmitted fromthe ball of the jar to the can, which renders it electro-positive ; and a considerable portion of this charge istransmitted to the resinous cake, spreading outwardsby its repulsive attribute, on every side of the can ;and when the latter is removed by its glass handle,the electric fluid advances inwards also, until theforces on opposite sides of the centre, balance oneanother, and completely arrest the fluid’s furtheradvance.

The cake being thus differently electrized on twoparts of its surface, is placed in a vertical position ;and then standing at a considerable distance, I projecttowards it the mixed powers from the spring bag,