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.
The experiment which proves this is made in the following manner. The amputated limb of an animal being placed upon a table, let the operator hold with one hand the principal nerve, previously laid bare, and in the other let him hold a piece of zinc; let a small plate of lead or silver be then laid upon the table, at some distance from the limb, and a communication be formed, by means of water, between the limb and the part of the table where the metal is lying. If the operator touch the piece of silver with the zinc, contractions will be produced in the limb the moment that the metals come into contact with each other. The same effect will be produced if the two pieces of metal be previously placed in contact, and the operator touch one of them with his finger. This fact was discovered by Mr. William Cruikshank.