An electric exhibiting the appearances above mentioned, is said to be excited, and some of them, particularly the tourmaline, are excited by heating and cooling, as well as by friction. It appears, however, that excitation consists in the mere transferring of electricity from one substance to another, and that the great source of electricity is in the earth. On this account it is necessary to the considerable excitation of any electric, that the substance against which it is rubbed (hence termed the rubber) have a communication with the earth, by means of conductors; for if the rubber be insulated, that is cut off from all communication with the earth by means of electrics, the friction has but little effect.
When insulated bodies have been attracted by, and brought into contact with, an excited electric, they begin to be repelled by it, and also to repel one another; nor will they be attracted again till they have been brought into contact with some conductor communicating with the earth; but after this they will be attracted as at first.
If conductors be insulated, electric powers may be communicated to them by the approach of excited electrics, or the contact of other electrified bodies. They will then attract light bodies, and give sparks, &c. like the excited electrics themselves.
When electricity is strongly communicated to insulated animal bodies, the pulse is quickened, and perspiration increased; and if they receive, or part with, their electricity on a sudden, a painful sensation is felt at the place of communication. But what is more extraordinary, is, that the influence of the brain and nerves upon the muscles seems to be of an electric nature.
This is one of the last and most important of all philosophical discoveries. I shall, therefore, give the result of all the observations that have hitherto been made on the subject, in a series of propositions, drawn up by an intelligent friend, who has given much more attention to it than I have done.
1. The nerve of the limb of an animal being laid bare, and surrounded with a piece of sheet lead, or of tinfoil, if a communication be formed between the nerve thus armed and any of the neighbouring muscles, by means of a piece of zinc, strong contractions will be produced in the limb.
2. If a portion of the nerve which has been laid bare be armed as above, contractions will be produced as powerfully, by forming the communication between the armed and bare part of the nerve, as between the armed part and muscle.
3. A similar effect is produced by arming a nerve and simply touching the armed part of the nerve with the metallic conductor.
4. Contractions will take place if a muscle be armed, and a communication be formed by means of the conductor between it and a neighbouring nerve. The same effect will be produced if the communication be formed between the armed muscle and another muscle, which is contiguous to it.
5. Contractions may be produced in the limb of an animal by bringing the pieces of metal into contact with each other at some distance from the limb, provided the latter make part of a line of communication between the two metallic conductors.