The process by which these wonderful appearances are produced consists in effecting, by means of the Galvanic apparatus, a communication between a nerve and a muscle, in any part of an animal body. The part of the animal upon which the experiment is to be performed is denominated the _animal arc_: and the Galvanic instruments which form the communication between the muscle and the nerve, are called the _excitatory arc_. This latter generally consists of three pieces; one fixed to the muscle, another to the nerve, and a third forming a communication between both. This last, called the _communicator_, may be made of the same metal with either of the others, or be different from both. The best communicators or conductors, are the following.—The list begins with the most perfect. Malleable platina. Silver. Gold. Quicksilver. Copper. Brass. Tin. Lead. Iron. The human body. Salt water. Fresh water. The metallic ores are not so good conductors as the purified metals, and their conducting power varies, according to the nature of the ores. The metallic salts are tolerably good conductors. Dr. Valli observed that human bodies are not all equally good conductors. Out of four persons in a company, he found that when two of them formed the circuit of communication between the nerve and muscles of a frog, the motions took place very readily. When the third person formed the circuit, the motions were very weak; but that, when the fourth person formed the communication, no motion took place. This experiment, he adds, was often repeated with the same success. The effect however may be owing to the different dryness of the skin. Vitriolic acid, and even alcohol, appear to conduct the Galvanic influence rather better than water. The veins and arteries are not so good conductors as the nerves; for when a blood-vessel forms part of the circuit of communication, the contractions will take place only when ramifications of the nerves are adhering to it, and if these be carefully separated, the motion will not take place. The same thing may be said of the tendons, the bones, and the membranes; for when either of those parts is separated from the body, and is introduced into the circle of communication between the muscles and nerves of a prepared frog, no motion will ensue; excepting, indeed, when those parts are full of moisture and in immediate contact with the nerve. CHAP. III. _A Description of the Galvanic Trough and Pile._