Volta constructed the pile which bears his name, on the assumption that “every two heterogeneous bodies form a galvanic circle or arc in which electricity is generated.” The “pile” consisted of a number of discs of zinc and copper separated by discs of card soaked in water. This combination of metals separated by a bad conductor, developed considerable electricity, the “positive” going to the zinc at the top, and the “negative” turning to the opposite end. By touching the zinc and copper extremities simultaneously with wetted fingers we shall experience a shock. “I don’t need your frog,” Volta said, when he had proved his theory; “give me two metals and a moist rag, and I will produce your animal electricity. Your frog is nothing but a moist conductor, and in this respect it is inferior to my wet rag!” [Illustration: Fig. 219.—Voltaic Pile.] After this discovery the theory of animal electricity died away for many years, till in 1825, Nobili, and afterwards Matteucci, proved the existence of galvanic currents in muscles. After Volta had succeeded in obtaining a shock from his “pile,” he proceeded to the construction of another instrument, or rather apparatus, which he denominated “Couronne des tasses” (fig. 220). It consisted of a series of small glasses containing water or a saline solution. He then procured a number of “metallic arcs,” partly composed of zinc and partly of copper; these were inserted into the glasses, so that every glass contained the zinc of one and the copper of another arc, not in contact, but one at the right hand the other at the left. The electro motion, supposed to be the primary cause of the galvanic action, was thus produced as well as from the “pile.” The principle was just the same in both apparatus, the metals being divided by the water in one case, and by a wet card or cloth in the other. Volta, in 1800, addressed to the Royal Society his celebrated letter upon electricity excited by contact of conducting substances, and then the English philosophers proceeded to make further experiments. It was Fabroni of Florence who had just before suggested that chemical action was really the cause of the phenomena exhibited. Sir Humphrey Davy warmly advocated this theory, and made numerous experiments with the view to establish it. Nicholson, Carlisle, and Cruickshank also paid great attention to the subject. Volta, although he had laid the foundation, did not venture to build upon it. Messrs. Nicholson and Carlisle found the two kinds of electricity in the pile, the zinc being positive and the silver negative. They also found that the water was decomposed both in the circuit and in the body of the pile. Subsequently Cruickshank confirmed Nicholson’s observations, and made use of what is termed the “trough” apparatus. He found that hydrogen was emitted from the silver or upper end, and oxygen from the other. [Illustration: Fig. 220.—Volta’s couronne des tasses.]