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in exactly the same manner, produce a greater or less quantity of electricity. Thus also, by only heating or cooling the metals, the electricity may be varied in quantity and even in quality.

I am inclined to suspect, that different bodies have different capacities for holding the electric fluid, as they have for holding the elementary heat; if however the experiments relative to this subject be carefully tried, under all the variety of circumstances which the combination of the above-mentioned causes is capable of producing, I do not doubt but that all the phenomena observed in the preceding pages may hereafter be reconciled to one, or to a few, simple laws, which will at the same time assist the farther investigation of the science of electricity.

These experiments of Cavallo's could not have been made later than the year of publication, viz., 1795. While Cavallo says in his discussion of animal electricity that Volta suspects that the phenomena of muscle contraction which Galvani and he were studying might be caused by the contact of the two dissimilar metals which were used in making the connection between the muscle and the nerve in many of their experiments, yet Volta seems not to have actually experimented with contact electrification until after the publication of Cavallo's treatise, and then to have begun with the repetition of Bennett's experiments made ten years before.

There seem to be many diverse statements as to how Volta arrived at his theory of contact electricity, but his own story of it is given in a so-called letter to Dr. Gren, which was published in volumes III. and IV. of Gren's Neues Journal der Physik, in the years 1796-98. These journals are not accessible to the present writer, so their exact date can not be given, though Vol. IV. was concluded in 1798. Volta's letters to Gren are translated in the Philosophical Magazine of 1799, from which the extracts here given are quoted.

In the first part of Volta's letter, which was published in Vol. III. of Gren's Journal, Volta says:

The contact of different conductors, particularly the metallic, including pyrites and other minerals as well as charcoal, which I call dry conductors, or of the first class, with moist conductors, or conductors of the second class, agitates or disturbs the electric fluid, or gives it a certain impulse. Do not ask in what manner: it is enough that it is a principle, and a great principle. This impulse, whether produced by attraction or any other force, is different or unlike, both in regard to the different metals and to the different moist conductors, so that the direction, or at least the power, with which the electric fluid is impelled or excited, is different when the conductor $$A$$ is applied to the conductor $$B$$, and to another $$C$$. In a perfect circle of conductors, where either one of the second class is placed between two different from each other of the first class, or, contrariwise, one of the first class is placed between two of the second class different from each other, an electric stream is occasioned by the predominating force either to the right or to the left—a circulation of this fluid, which ceases only when the circle is broken, and which is renewed when the circle is again rendered complete.