Volta experimented with a great many different metals, and he soon was able to show that the electricity produced by contact can be proved to exist by one of the measuring instruments called electroscopes, which he had himself done much to make more delicate and sensitive. He found also that metals behaved very differently, some of them giving much electricity, and others little. Zinc and copper were found to be a good combination. The zinc was found after contact with copper to have the same kind of electrical condition as glass when rubbed with silk. That kind of electricity was at that time usually called glassy or “vitreous” electricity. The copper was found to be in the same kind of electric state as sealing wax or resin after they have been rubbed with wool or catskin. This was called “resinous electricity.” The two kinds of electricity were known to attract each other, and when they were allowed to combine, they rushed together in a spark, and completely disappeared, the one just neutralising the other. Following the practice of arithmetic and algebra, the two electricities were also called positive and negative. Franklin thought that the positive electricity was the real electric fluid, and that the negative electricity was simply the want of that fluid. It was a mere guess of his, and he might have guessed just the other way. If he had done so, he would have been much more correct, for it has since been found out that the resinous electricity is best considered as the real fluid, and vitreous electricity as due to a want or shortage of that fluid. However, at that time there was no way of telling what was the real electric fluid, and even the number of electric fluids which existed. Volta was sure of the existence of vitreous and resinous electricity, but in addition to this Galvani maintained the existence of a third fluid, which he called animal electricity. After many more experiments, Volta was able to arrange the chief metals in a row or series, in such a manner that any metal, when in contact with any other metal further on in the series, would acquire vitreous electricity. This series was the following: Zinc, lead, tin, iron, copper, silver, gold. He found that zinc could not be made to acquire resinous electricity by contact, nor could gold be made to acquire vitreous electricity. The strongest effect was obtained by the contact of metals as far apart in the series as possible. The strongest effect of all was obtained by touching zinc with gold. [Illustration: Fig. 50.--Volta’s Pile.]