pad, so that the friction may take place under a suitable pressure and with sufficient speed. The machine is also fitted with spiked combs for taking off the negative charge from the pad and the positive from the glass, and generally there is some contrivance added for storing the charges, or one of them. Machines of this kind are very inefficient, and as they have within our generation been superseded by much more efficient machines working on a different principle, which are treated in the fourth chapter, we need not discuss them in detail.
The frictional machine was, however, up to the year 1789 the only practical means of producing such electrification as the physicist of those days required for his experiments. In that year there came a change. L. Galvani, Professor at the Bologna University, found that electric effects could be produced in animal tissue, if this were put into contact with two different metals, in his case copper and iron. His experiment with the frogs' legs is so well known that it would be wasting space to describe it here. Galvani looked for the cause of the phenomena observed in the tissue and not in the metals. In this he was mistaken. He assumed the existence of
