Whilst Galvani persisted in this mistake until his death, Volta realized that the source of the electric force, as in the first of Galvani's observations, must still be sought outside the specimens, and himself rightly attributed it to the contacting metals. Guided by this hypothesis, Volta started systematic research into the Galvanic properties of metals, and presently succeeded in producing electricity once more from purely mineral substances, namely from two different metals in contact with a conductive liquid.