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this experiment that the attraction of electricity is subject to the same laws with that of gravitation, and is therefore according to the squares of the distances; since it is easily demonstrated that were the earth in the form of a shell, a body in the inside of it would not be attracted to one side more than another?"

This brilliant inference seems to have been insufficiently studied by the scientific men of the day; and, indeed, its author appears to have hesitated to claim for it the authority of a complete and rigorous proof. Accordingly we find that the question of the law of force was not regarded as finally settled for eighteen years afterwards.^[1]

By Franklin's law of the conservation of electric charge, and Priestley's law of attraction between charged bodies, electricity was raised to the position of an exact science. It is impossible to mention the names of these two friends in such a connexion without reflecting on the curious parallelism of their lives. In both men there was the same combination of intellectual boldness and power with moral earnestness and public spirit. Both of them carried on a long and tenacious struggle with the reactionary influences which dominated the English Government in the reign of George III; and both at last, when overpowered in the conflict, reluctantly exchanged their native flag for that of the United States of America. The names of both have been held in honour by later generations, not more for their scientific discoveries than for their services to the cause of religious, intellectual, and political freedom.

The most celebrated electrician of Priestley's contemporaries in London was the Hon. Henry Cavendish (b. 1731, d. 1810), whose interest in the subject was indeed hereditary, for his father, Lord Charles Cavendish, had assisted in Watson's experiments of 1747.^[2] In 1771 Cavendish^[3] presented to the Royal Society an "Attempt to explain some of the principal phenomena of Electricity, by means of an elastic fluid." The hypothesis

1. In 1769 Dr. John Robison (b. 1739, d. 1805). of Edinburgh, endeavoured to determine the law of force by direct experiment, and found it to be that of the inverse 2.06^th power of the distance.
2. Phil. Trans. xlv, p. 67 (1750).
3. Phil. Trans. Ixi, p. 084 (1771).

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