It occurred to Volta that perhaps the strongest effect might be obtained by combining several pairs of the same metals in succession. He usually used zinc and copper, because silver and gold were too expensive. He fixed four glass rods in an upright position, and cut a large number of round plates of zinc, copper, and leather or cardboard. The leather or cardboard he soared in salt water. He laid a plate of copper at the bottom between the four glass rods. On this he placed a plate of zinc, then a piece of cardboard, then a plate of copper, then a plate of zinc, then another piece of cardboard, and so on until he had made a big pile of such combinations. This pile is known to this day as Volta’s Pile. Volta found that such a pile gave a little spark on attaching a wire to the top of it, and bringing the other end of the wire in contact with the bottom of the pile. [Illustration: Fig. 51.--Volta’s Crown of Caps.] He therefore called such a pile an “artificial electric organ,” in order to direct attention to its similarity to the electric organ of a cramp fish. The pile did not last long. It either dried up, or if there was too much water, the water ran down the sides and interfered with the proper contact of the metals. Volta, however, improved it by using salt water in a row of cups in order to conduct the electricity from one pair of metals to the next. Instead of using round plates, he used the metals in the shape of flat bands, soldering together a copper band and a zinc band, and arranging the cups in the form of a crown. The spark obtained with the pile, or crown of cups, was not sufficient to prove that the electricity produced by it was the same as that furnished by the electric machine. But the proof was not long in coming. In the year 1789, water had been separated into the two gases of which it is composed by conducting the discharge of a Leyden jar through the water a great many times. The same effect was found to result also from the action of the electric current obtained from one of Volta’s piles. This coincidence was discovered accidentally.