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68 by a chain with the earth, while at the same time positive electricity from the prime conductor of an electrical machine is communicated to the inside knob.

The positive electricity will accumulate on the inside coating with which the knob is connected. It will then decompose the two electricities of the outside coating, driving the positive electricity to the earth, and there dissipating it, but attracting the negative to itself. There will thus be positive electricity on the inside, and negative on the outside coating. These two electricities may be compared to two hostile armies watching each other, and very anxious to get together, while, however, they are separated from one another by means of an insurmountable obstacle. They will thus remain facing each other, and at their posts, while each side is, meanwhile, being recruited by the same operation as before. We may by this means accumulate a vast quantity of opposite electricities on the two coatings of such a jar, and they will remain there for a long time, especially if the surrounding atmosphere and the glass surface of the jar be quite dry. When, however, electric connection of any kind is made between the two coatings, the electricities rush together and unite with one another in the shape of a spark, while if the human body be the instrument of connecting them a severe shock will be felt.

90. It would thus appear that, when two bodies charged with opposite electricities are brought near each other, the two electricities rush together, forming